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

pyroclastic rock
consolidated volcanic fragments that have been extruded explosively and deposited by settling (like sediment) either on land or in water.

pyroelectric
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

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

slate
a microcrystalline metamorphic rock that has rock cleavage -- i.e., it splits readily into

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 San…

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

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.

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.

H. (hardness)
resistance to scratching or abrasion.