Definitions given in this Glossary indicate how each term is used on this web site -- i.e., none of these definitions should be considered either hard-and-fast or all-inclusive. With a few exceptions, names of minerals, rocks and fossils are not included.
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Aaron's breastplate: See breastplate.
acidic:adjective used to describe magmas that contain more than 60 percent SiO2 and also to igneous rocks and masses solidified from such magmas. The rocks -- e.g., granites and granodiorites -- are typically light colored and have relatively low specific gravities (i.e., within the range of 2.45-2.70).
adularescence: bluish sheen exhibited by some feldspars -- e.g., moonstone -- when light is reflected from certain directions; cf. labradorescence.
alaskite: a granitic rock that contains less than 5 percent of dark-colored minerals.
alkali feldspar: "sack term" applied in some schemes of petrographic nomenclature, especially those for igneous rocks, to potassium feldspar (microcline or orthoclase), sodium-rich plagioclase (albite) and/or their perthitic combinations. In the vernacular, it is frequently used as a synonym for potassium feldspar.
alkalic rock: quartz-free igneous rock that consists largely of feldspars and feldspathoids and commonly contain other alkali-rich minerals, but lack quartz.
alkaline complex: rock mass made up largely or wholly of alkalic igneous rocks.
alluvial: relating to stream (i.e., running water) activities or deposits.
alluvium: deposits from running water.
amygdule (adj. amygdaloidal): a filled, commonly almond-shaped, vesicle (q.v).
aphanite: crystalline rock in which the individual grains are so small they cannot be distinguished by the naked eye or even with the aid of a handlens.
arenaceous: sandy -- i.e., consisting in a noteworthy part of quartz sand.
argillaceous: clayey -- i.e., containing abundant clay.
ashlar: slab, commonly rectangular in shape, of dressed stone for facing either exterior or interior walls.
assembled stone: constructed gemstone that consists of two or more materials that are bound or fused together; constituent materials may be any combination of natural or synthetic minerals or rocks or manufactured materials (e.g., glass, plastic, or ceramic). Assembled stones that consist of only two materials are frequently called doublets, and those of three materials are termed triplets.
basic: adjective used to describe magmas that contain 44 to 52 percent SiO2 and also to the igneous rocks and masses solidified from such magmas. The rocks -- e.g., gabbros and basalts -- are typically dark colored and have relatively high specific gravities (i.e., within the range 2.94-3.25).
bedrock: the continuous solid rock that is exposed or directly beneath unconsolidated overburden.
botryoidal: said of the habit of minerals whereby they comprise grape-shaped masses.
breastplate: a highly embroidered, woven fabric that in ancient times was an integral part of the Jewish High Priest's vestments. Suspended by a neck chain so as to hang in front of his chest, it was rectangular and had 12 precious stones – three in each of four tiers, each stone representing one of the 12 tribes of ancient Israel -- mounted on it. The prototype was apparently fashioned for Aaron, who is widely considered to have been the first High Priest -- thus, the many references to "Aaron's breastplate." The "High Priest's breastplate" -- "breast piece" in some translations -- also widely referred to in the literature, apparently refers to modified versions of Aaron's breastplate worn by his High Priest successors. Difficulties associated with translations of the pertinent sources of information have led to disagreements both as to what stones were present and in what positions they occupied on these breastplates. (See, for example, different translations of Exodus 28:17-21 and Douglas(ed.) 1980, v.2, p.785.) These differences account for my less than explicit statements about the gemrocks mounted on both Aaron's and the High Priests' breastplates.
brittle: adjective applied to minerals and rocks that rupture easily.
buffalo stones: fossils once considered by American Indians of the western planes to be charms that would insure their having success while hunting buffalos.
cabochon: name given cut and polished gemstones that have convex tops; their typical shapes, when viewed from the top, are ovals, including circles.
carbonaceous: containing a noteworthy part of so-to-speak amorphous carbon and/or some solid organic hydrocarbon(s); so-designated rocks are commonly black or dark gray.
celt: prehistoric stone tool, such as an axehead.
chatoyant: said of changing luster that resembles that of a cat's-eye or that of a spool of silk thread moved under reflected light.
clastic: term applied to sediments and their lithified rock equivalents that consist of fragments of pre-existing rock; detrital is a frequently used synonym.
clasts: "sack term" applied to fragments (e.g., pebbles, cobbles and boulders), usually to those within aggregates, such as gravels, conglomerates, and sedimentary breccias.
cleavage: property of some crystalline substances to break along plane surfaces the positions of which are controlled by the substance's internal structure.
coke: A combustible material derived from agglomerating coal... It is produced by carbonization -- i.e., heating coal enough to drive the coal's volatile matter. Coke is gray, hard, and porous, and as a fuel is practically smokeless. Although iIt occurs in nature, most of it is manufactured." (Bates and Jackson, 1987). This word is also used as a verb to describe the process -- i.e., some coal will coke whereas other coal will not when submitted to the same, widely employed processes.
conchoidal: smooth, concave, shell-like fractures characteristic of, for example, glass and quartz.
concretion: a spheroidal, ellipsoidal (commonly oblate) or irregularly shaped mass, typically developed within sediments by localized deposition around a nucleus.
country rock: rock intruded by magma and now surrounding the igneous rock mass formed by consolidation of that magma; also, more widely applied by some geologists to all rock formations cropping out or under the unconsolidated mantle of a given area.
cryptocrystalline: said of mineral materials that consist of individual mineral units that are distinguishable only under a microscope; cf. microcrystalline.
dendrite: an arborescent -- i.e., irregularly branching like a tree -- mineral growth (commonly of some manganese oxide).
density: see specific gravity.
detrital: synonym of clastic.
diabase: rock that is, in essence, a fine-grained gabbro.
diagenetic: pertaining to changes -- such as recrystallization and replacement -- that take place in sediments after their deposition but before, and commonly contributing to, their conversion to rock; term is also applied rocks that consist largely of materials formed in response to such changes.
diaphaneity: the state or quality of transmitting light; degrees of diaphaneity include transparent, translucent and opaque.
dripstone: mineral matter deposited from a dripping solution; this is an overall term for formations such as stalactites and draperies that are formed in caves; cf. speleothem.
effervescence: to bubble or hiss when, for example, dilute HCl (hydrochloric acid) is put on calcite or limestone.
erratic: see glacial erratic.
extrusive: relating to lava -- i.e., magma ejected out onto the Earth's surface; also applied to volcanic rocks formed from lava.
fault: a rock fracture with a lateral displacement of the rocks on one side of the break with respect to those on the other side. The locus of fracture, commonly a zone up to several meters thick, is generally called a fault zone.
fissility: see shale.
flint: the name frequently given to chert, especially that which is dark gray to nearly black and dully lustrous; in the past it was used rather widely for artifacts.
fluorescence: visible light emitted from a material while it is exposed to ultraviolet radiation ("black light").
foliation (adj. foliated): the streaked or banded appearance of a metamorphic rock that is the manifestation of a preferred orientation of platy or rodlike constituent mineral grains, the orientations of which resulted from directional pressure during their growth.
friable: said of rocks that are easily crumbled -- e.g., a poorly cemented sandstone.
gabbro: igneous phanerite typically consisting of 50 to 90 percent dark-colored plagioclase, less than 5 percent quartz and 35 to 65 percent varietal minerals (augite ± another pyroxene ± olivine).
glacial erratic: a glacially transported and deposited stone, commonly of boulder or cobble size.
glass: liquid with its viscosity so high that it appears solid, even though its constituent atoms and ions do not have a regular arrangement like those of crystalline substances.
glomeroporphyritic: adjective used to describe porphyries characterized by complex clusters of two or more phenocrysts.
gneiss: a roughly foliated or banded metamorphic rock consisting largely of granular minerals with its platy and rodlike minerals defining the foliation; many gneisses are of granitic composition.
granite: phaneritic igneous rock typically made up of quartz - 20-60 percent, alkali-feldspar > plagioclase feldspar, and 5-20 percent dark minerals (typically biotite ± muscovite ± hornblende).
granodiorite: phaneritic igneous rock typically consisting of quartz - 20-60 percent, plagioclase > alkali-feldspar, and 5-25 percent dark minerals (typically biotite and hornblende).
granular: adjective usually used to describe rocks consisting largely of roughly equidimensional grains.
greenstone: omnibus term variously applied to green gemrocks, such as certain metamorphosed basalt, jade (both jadeitic and nephritic), serpentinites, and verde antique.
groundmass: see porphyry.
H. (hardness): resistance to scratching or abrasion.
The numbers -- 1 to 10 -- used in this book, and widely by mineralogists throughout the world, refer to the scale based on relative hardnesses of common minerals that was devised by Friedrich Mohs (1773-1839), a German crystallographer. In this scale, the minerals are ranked in a sequence of relative hardness based on their ability to scratch or be scratched by one another. That is to say, minerals (etc.) scratch minerals (etc.) with lesser hardnesses and can be scratched by materials of greater hardnesses. Thus, for example, a mineral with a hardness of 7½ would scratch quartz (or any softer material) or be scratched by topaz (or any harder material)...
Mohs Hardness scale:
<fingernail (H. ~2½)
<copper coin (H. ~ 3½)
<jackknife and window glass (H. ~ 5½)
Attention is directed to the remarks in the INTRODUCTION about how hardness should be determined and cautions that should be observed when checking hardnesses of most gemrocks!!
High Priest's breastplate: See breastplate.
hololith: name given rings,
etc. fashioned froma single piece of gemrock -- i.e., they
have no setting or any other material, such as the "string" of beads,
hold them together.
hornstone: alternative name sometimes given to chert; also applied by some to other rocks such as hornfels, which is not treated in this document.
hydrothermal activity: deposition and changes in rocks and/or minerals dependent upon the actions of heated aqueous solutions, the water of which may be of magmatic, meteoric or connate origin; processes such as alteration, replacement, and ore deposition are representative.
igneous: said of processes involving magma and of the rocks formed by cooling and consolidation of magma.
impactite: rock, typically glassy -- and in some cases vesicular -- and/or partly or wholly fine grained crystalline, produced when and where some meteors, particularly large ones, have collided with the earth.
intrusive: adjective applied to igneous masses consolidated beneath the earth's surface.
iridescence: exhibition of multicolored reflections such as the "play of colors" given by films of oil on water.
labradorescence: phenomenon similar to adularescence, in some cases involving relatively larger areas, each marked by a single hue -- typically yellowish, greenish, bluish, brown or golden -- that tends to change gradually as the mineral is moved when viewed under reflected light.
lava: magma extruded out on to the earth's surface.
lignite: brown coal, widely considered to be transitional between peat and bituminous coal.
lithification: conversion to rock.
luminescence: emission of light by a substance as the result of some external stimulus.
luster: appearance of a surface in reflected light.
magma: molten or partially molten rock material that has mobility or at least potential mobility that is dependent upon the presence of the molten rock constituent.
massive: term sometimes referred to rocks of virtually homogeneous character.
matrix: the relatively fine grained mineral material that surrounds relatively larger fragments of other mineral matter; two examples are the fine grained portions of breccias and the non-turquoise portions of "turquoise matrix."
metamorphic: term applied to rock resulting from the transformation of preexisting rock in response to elevated temperature and/or pressure, sometimes accompanied by changes in chemical environment (e.g., compositional changes of percolating fluids); the processes involved are termed metamorphism.
metasomatism: metamorphism caused chiefly by introduction of fluids and/or ions. The adjective metasomatic is used to described rocks that have undergone such changes resulting from such activities.
microcrystalline: said of a rock consisting of mineral grains that are distinguishable only under a microscope; cf. cryptocrystalline.
migmatite: macroscopically composite rock typically made up of a dark-colored amphibolite or biotite gneiss intimately mixed with a light-colored rock of granite to granodioritic composition.
monzonite: a phaneritic igneous rock that consists of nearly equal percentages of alkali-feldspar and plagioclase feldspar and 20-50 percent dark minerals, typically biotite and hornblende.
mylonite: rock formed by extreme microbrecciation within a fault zone.
nodule: term widely used as a near synonym for concretion.
oolitic: consisting of small, nearly spherical, typically calcium carbonate masses that resemble fish roe -- e.g., the Indiana limestone, which is used widely as a building stone and ashlar; this term is, however, also applied to other rocks, such as chert, that are made up of such spheroidal masses.
opalescence: pearly or milk-like reflections that appear to come from within any material so described.
orbicular granite: term applied to granites (etc.) that consist of diversely constituted spheroidal nodules, called orbicules, enclosed within matrices that have essentially normal textures; the orbicules, which range up to several centimeters in diameter, are typically made up of concentric shells in which one or more minerals predominate. For an illustration, see Dietrich and Skinner (1979), Figure 4-14, page 123.
ore: any rock from which one or more valuable minerals can be recovered at a profit.
pearly: said of luster that looks like pearl.
pegmatite: any exceptionally coarse-grained igneoid rock, typically occurring as lenses or irregular masses that consist largely of quartz, alkali feldspar, and mica; the adjective pegmatitic is sometimes applied to coarse grained igneoid rocks no matter what their composition.
perlite: a partially altered glassy volcanic rock with a chemical composition similar to that of obsidians (i.e. of granites) that is characterized by closely spaced concentric cracks; most perlites are light gray in color.
perthite: an intimate mixture of either microcline or orthoclase and a sodium-rich plagioclase (commonly albite).
phanerite: term frequently used to describe igneous rocks the specific minerals of which can be identified with the naked eye.
phenocryst: relatively large, conspicuous crystals surrounded by a finer grained matrix (groundmass) in igneous rocks called porphyries.
phosphorescence: luminescence that continues after the source of excitation has been removed; a so-to-speak afterglow.
pietra dura (Italian, hard rock/stone): term widely applied to decorative inlays -- typically consisting of diverse gem materials such as agate, alabaster, carnelian or lapis lazuli, and commonly also mollusk shell (etc.) -- in jet, marble, shale or some other relatively soft stone, which becomes the so-to-speak background. The resulting pieces, produced since the late 16th century, have been used in jewelry (e.g., pendants) and panels of diverse sizes (e.g., for "keepsake" boxes, altars, and table tops). "Florentine mosaic" is a sometimes used synonym based on the fact that pietra dura was a specialty of artisans working in the Grand Duke Ferdinando de Medici's Uffizi (now an art gallery) in Florence, Italy. (See The Grove dictionary of art. 1999. [electronic resource: www.groveart.com/tdaonline/index.html]. New York: Grove's Dictionaries, Inc.)
plutonic: rocks formed at great depths; most rocks so designated are igneous, metamorphic, or migmatitic.
pneumatolysis: rock alteration caused by gases widely thought to be genetically related to magma; cf. metasomatism.
polysynthetic twinning: closely spaced twinning, involving three or more -- typically several score -- individuals twinned on parallel composition planes and in accordance with the same twinning "law." Polysynthetic twinning, which is a characteristic of many plagioclase feldspars, is commonly discernible macroscopically; it appears as a series of parallel lines on certain surfaces (see, for example, Dietrich and Skinner, 1979, Fig. 2-8, p.39).
porphyry: igneous rock in which relatively large and conspicuous grains, called phenocrysts, are surrounded by a finer-grained matrix, generally called the groundmass.
pseudomorphism:process(es) whereby one mineral takes on the form of another mineral. The mineral having such form is called a pseudomorph and is said to be after the precursor mineral -- e.g., limonite pseudomorph after pyrite. Processes generally thought to be involved in such replacement are alteration, incrustation, or paramorphism (which amounts to the change of a mineral with a given chemical composition and internal crystalline structure to a different mineral with the same composition but different internal structure -- e.g., aragonite to calcite)
pyroclastic rock: consolidated volcanic fragments that have been extruded explosively and deposited by settling (like sediment) either on land or in water.
said of any material that generates an electric charge as the
result of a change of temperature -- e.g., heating.
reniform: said of kidney-shaped mineral masses.
replacement: any chemical process thought to involve essentially simultaneous removal of one mineral and deposition of another in its place.
resinous: having a luster resembling that of natural resin.
reverse intaglio: name applied, especially in the marketplace, to carvings into the back sides of transparent and translucent stones used in jewelry such as brooches and pendants. [ I have a real problem with this term -- one that relates to semantics and admittedly is nitpicking: When one considers the widely accepted definitions for cameo and intaglio, this designation tends to equate with cameo, eh(?). This false impression could, in my opinion, be removed by replacing the adjective reverse with backside, underside, or perhaps even posterior. Furthermore, venders frequently indicate that these and several similarly fashioned gemstones and decorative pieces were carved; this is misleading so far as how many of them were shaped: Carving is widely accepted as involving cutting, either wholly or for the most part, by utilizing sharp tools, whereas several of the so-described items were produced by etching, lasering and/or other means rather than by carving. I f an inclusive term is desired, perhaps engrave (etc.) would serve the purpose.
rhyodacite: the aphanitic
equivalent of granodiorite.
rhyolite: the aphanitic equivalent of granite.
sandstone: a clastic sedimentary rock that is lithified sand.
schiller: a phenomenon whereby a metallic-like shimmering is seen just below the surface when the so-characterized material is viewed from certain directions under reflected light; sometimes described only as a "play of color."
schist: well-foliated metamorphic rock consisting of a significant percentage of one or more platy minerals such as one of the micas and/or chlorites.
sectile: said of minerals that can be cut with a knife without breaking off in undesired pieces.
sedimentary: said of rock consisting of consolidated sediment; also applied to processes related to the formation of such deposits.
serpentinite: a metamorphic rock that consists largely of the mineral serpentine.
shale: a sedimentary rock made up largely of clay particles arranged so the rock has fissility -- i.e., will split readily parallel to bedding laminae.
siliceous: consisting in a noteworthy part of silica, typically quartz.
silicified: replaced or impregnated by some form of silica.
silky: said of a luster resembling that of silk cloth.
simulant: a natural or artificially produced material that resembles another material -- e.g., chalcedony, either naturally or dyed green, and green glass are simulants of jade; cf. synthetic.
sinter: process of heating (without melting) whereby a coherent mass is made from many smaller particles.
skarn: a metamorphic rock formed by the introduction of magmatic fluids of diverse composition into, for example, limestones and dolostones that were adjacent to or near the contacts of the magma; the processes responsible for the formation of these rocks are widely termed contact metasomatism or pneumatolysis.
slate: a microcrystalline metamorphic rock that has rock cleavage -- i.e., it splits readily into slabs.
specific gravity (= S.G. in text): this number, termed density in some publications, is the ratio of the weight of a substance to the weight of an equal volume of water. There are a number of ways it can be determined. Two follow:
1. Weigh the object in air (Wa), weigh it again while it is immersed in water (Ww), and substitute those values in the following equation:
S.G. = Wa / Wa - Ww.
Although weighing some pieces -- e.g., certain pieces of rough material and carvings -- in water may be difficult, it usually can be done. Two main cautions are a) be sure that no air bubbles remain around or within the object being weighed and b) if the material is porous, either coat it with a thin coating of, for example, paraffin or carry out the weighing in water quickly(!).
2. The suspension method, which involves the direct comparison of the material in question with heavy liquids having known (or subsequently determined) specific gravity values. This method involves immersing the material being checked (e.g., gemrock) in a suitable liquid and determining whether the material sinks, floats, or remains suspended at virtually any level where it is placed.
Suitable liquids for use with many gemrocks are:
Bromoform, CHBr3 -- S.G. = 2.89
Acetylene tetrabromide (tetrabromoethane), C2H2Br4 -- S.G. = 3.32
Clerici solution (saturated aqueous solution of equal amounts of thallous malonate and thallous formate) -- S.G. = 4.2 (at room temperature).
Fortunately, the densities (S.G.s) of the first two fluids, which are organic, can be lowered by diluting them with acetone (S.G. = 0.79) or xylene (S.G. = 0.86), and the density of clerici solution can be lowered by diluting it with water (S.G. = 1.0). The resulting densities (i.e., specific gravity values) of the diluted fluids can be checked by immersing materials having known S.G. values.
>>>>> NOTE: Clerici solution must be used with great caution because it is both toxic and corrosive.
speleothem: overall term for cave deposits of chemical origin.
spherulite: spheroidal masses typically made up of units or complexes that radiate from the center of the mass so that sections through the center resemble spoked wheels.
stalactite: a roughly conical or icicle-shaped speleothem that hangs down from the roof of a cave; sometimes termed pendant.
stalagmite: a roughly conical shaped speleothem that rises from the floor of a cave.
stratigraphic unit: mappable rock unit that has one or more characteristics -- e.g., the composition of its predominant rock constituent -- that distinguish it from overlying and underlying units. Such units have been given binomial names – according to specific, widely accepted codes (e.g., Salvador, 1994) – that consist of a geographic name followed by the name of the predominant rock constituent. The geographic name indicates the place where exposures of the rock were first described. An example is the Potsdam Sandstone, which was named for an exposure of Cambrian sandstone in Potsdam, St. Lawrence County, New York. These names are used in geological reports, on geological maps, and elsewhere in the scientific literature.
streak: color or appearance of a powder (etc.); it is usually observed after a mineral or other material is drawn across a plate of unglazed white porcelain or a touchstone (see in JASPER entry).
subtranslucent: translucent only at edges of or in thin slivers of a mass; same as the semitranslucent of some writers.
subtransparent: imperfectly or partially transparent; same as the semitransparent of some writers.
supergene: adjective applied to ore minerals and ores generally thought to have been formed by downward moving aqueous solutions; the term supergene enrichment is often given zones containing noteworthy amounts of minerals, notably copper, so-formed.
syenite: phaneritic igneous rock the light colored mineral content which is wholly or largely alkali feldspar plus a dark mineral content (typically hornblende) that ranges between 10 and 35 percent.
synthetic: as applied to gemstones this term indicates any man-made material that is virtually the same as the natural material -- cf. simulant.
tectonic: adjective referred to structural (i.e., positional) changes of rocks, typically manifest by one or more kinds of deformation such as folding, faulting, and/or jointing.
tenacity: this relates to the cohesiveness of both minerals and rocks.
texture: geometric interrelationships among constituent mineral grains in a rock.
toughness: a property, frequently applied to materials such as jade, that differs from hardness in that it refers to a resistance to breaking -- i.e., breaking or chipping rather than scratching.
translucent: said of substances through which light will pass but not clear enough so something -- e.g., writing -- can be seen through them.
transparent: said of substances that are clear to the point that things can be seen through them.
trap rock: term sometimes applied to basalt and even to dolerite (=fine grained gabbro =~diabase).
triboluminescence: luminescence caused by striking; the "sparking" when rock crystal is sawed is an example.
ultrametamorphism: extremely high grade metamorphism that involves extremely highly elevated temperatures and/or pressures.
vein: a mineral-filled fracture in rock.
vesicle (adj. vesicular): a cavity, commonly spheroidal, formed by the expansion of a gas bubble during solidification of a magma; cf. amygdule.
vitreous: luster like that of the surfaces of broken glass.
volcaniclastic: "sack term" applied to clastic deposits that contain noteworthy volcanic materials and their lithified equivalents.
vulcanism: term given to processes whereby magma is extruded on to the earth's surface.
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