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Michigan University - Gemological terms
Category: Earth and Environment > Gemrocks
Date & country: 13/09/2007, USA
Words: 147

Aaron's breastplate
See breastplate.

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).

bluish sheen exhibited by some feldspars -- e.g., moonstone -- when light is reflected from certain directions; cf. labradorescence.

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 comple
xrock mass made up largely or wholly of alkalic igneous rocks.

relating to stream (i.e., running water) activities or deposits.

deposits from running water.

amygdule (adj. amygdaloidal)
a filled, commonly almond-shaped, vesicle (q.v).

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.

sandy -- i.e., consisting in a noteworthy part of quartz sand.

clayey -- i.e., containing abundant clay.

slab, commonly rectangular in shape, of dressed stone for facing either exterior or interior

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).

the continuous solid rock that is exposed or directly beneath unconsolidated overburden.

said of the habit of minerals whereby they comprise grape-shaped masses.

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.…

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.

name given cut and polished gemstones that have convex tops; their typical shapes, when viewed from the top, are ovals, including circles.


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.

prehistoric stone tool, such as an axehead.

said of changing luster that resembles that of a cat's-eye or that of a spool of silk thread moved under reflected light.

term applied to sediments and their lithified rock equivalents that consist of fragments of pre-existing rock; detrital is a frequently used synonym.

sack term applied to fragments (e.g., pebbles, cobbles and boulders), usually to those within aggregates, such as gravels, conglomerates, and sedimentary breccias.

property of some crystalline substances to break along plane surfaces the positions of which are controlled by the substance's internal structure.

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.

smooth, concave, shell-like fractures characteristic of, for example, glass and quartz.

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.

said of mineral materials that consist of individual mineral units that are distinguishable only under a microscope; cf. microcrystalline.

an arborescent -- i.e., irregularly branching like a tree -- mineral growth (commonly of some manganese oxide).

see specific gravity.

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.

synonym of clastic.

rock that is, in essence, a fine-grained gabbro.

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.

the state or quality of transmitting light; degrees of diaphaneity include transparent, translucent and opaque.

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.

adjective applied to igneous masses consolidated beneath the earth's surface.

to bubble or hiss when, for example, dilute HCl (hydrochloric acid) is put on calcite or limestone.

see glacial erratic.

relating to lava -- i.e., magma ejected out onto the Earth's surface; also applied to volcanic rocks formed from lava.

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.

see shale.

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.

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.

said of rocks that are easily crumbled -- e.g., a poorly cemented sandstone.

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.

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.

adjective used to describe porphyries characterized by complex clusters of two or more phenocrysts.

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.

omnibus term variously applied to green gemrocks, such as certain metamorphosed basalt, jade (both jadeitic and nephritic), serpentinites, and verde antique.

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).

phaneritic igneous rock typically consisting of quartz - 20-60 percent, plagioclase > alkali-feldspar, and 5-25 percent dark minerals (typically biotite and hornblende).

adjective usually used to describe rocks consisting largely of roughly equidimensional

see porphyry.

High Priest's breastplate
See breastplate.

name given rings, bracelets, etc. fashioned froma single piece of gemrock -- i.e., they have no setting or any other material, such as the 'string' of beads, to hold them together.

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.

said of processes involving magma and of the rocks formed by cooling and consolidation of

exhibition of multicolored reflections such as the 'play of colors' given by films of oil on water.

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.

magma extruded out on to the earth's surface.

brown coal, widely considered to be transitional between peat and bituminous coal.

conversion to rock.

emission of light by a substance as the result of some external stimulus.

appearance of a surface in reflected light.

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.

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

term sometimes referred to rocks of virtually homogeneous character.

xthe 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.'

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.

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.

said of a rock consisting of mineral grains that are distinguishable only under a microscope; cf. cryptocrystalline.

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.

rock alteration caused by gases widely thought to be genetically related to magma; cf. metasomatism.

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.

rock formed by extreme microbrecciation within a fault zone.

term widely used as a near synonym for concretion.

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.

pearly or milk-like reflections that appear to come from within any material so

any rock from which one or more valuable minerals can be recovered at a profit.

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.

said of luster that looks like pearl.

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.

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.

an intimate mixture of either microcline or orthoclase and a sodium-rich plagioclase (commonly albite).

term frequently used to describe igneous rocks the specific minerals of which can be identified with the naked eye.

relatively large, conspicuous crystals surrounded by a finer grained matrix (groundmass) in igneous rocks called porphyries.

luminescence that continues after the source of excitation has been removed; a so-to-speak afterglow.

rocks formed at great depths; most rocks so designated are igneous, metamorphic, or

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).

igneous rock in which relatively large and conspicuous grains, called phenocrysts, are surrounded by a finer-grained matrix, generally called the groundmass.

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 inte…