Amorphous—without definite molecular structure; not crystalline.
Baroque stone—an irregularly shaped, polished stone; usually applied
to tumbled stones.
Baroque pearl—an irregularly shaped pearl.
Brilliancy—reflecting much light; having brightness.
Brilliant cut—a mode of arrangement of facets commonly used on round
or oval stones. The standard American brilliant cut has 57 or
58 facets. Most diamonds of 5 or less carats are cut in this
manner.
Cabochon—a stone cut with a flat or convex upper surface; sometimes
faceted in part. Opal, star sapphire, and agate are stones
that are frequently cut in this style (fig. 2).
Cambrian—a division of geologic time, estimated to be the time from
550 to 440 million years ago; the oldest time division of the
Paleozoic era.
Carat—a unit of weight equal to ⅕ of a gram or 0.2 gram. One ounce
avoirdupois is equal to 141.75 carats.
Cleavage—the tendency of certain minerals to split in particular
directions yielding relatively smooth plane surfaces.
Conchiolin—an organic albuminoid substance found in pearls.
Conchoidal—a type of fracture having curved concavities or the
approximate shape of one-half of a bivalve shell. Glass has
excellent conchoidal fracture.
Cretaceous—a division of geologic time, estimated to be the time from
135 to 60 million years ago; youngest division of the Mesozoic
era.
Crown—that portion of a faceted gem above the girdle; the upper
portion of a facet-cut gem (fig. 6).
Cryptocrystalline—composed of very fine or microscopic crystals.
Crystal—the regular polyhedral form, bounded by plane surfaces, that
is assumed by a mineral under suitable conditions. Crystals
have definite external symmetry and internal molecular order.
Crystalline—possessing definite internal molecular order; not
amorphous.
Cubic—in the general shape of a cube. The isometric crystal system is
often called the cubic system.
Culet—the very bottom portion of a faceted gem; the point or line
formed by the intersection of the lowest pavilion facets (fig.
6).
Dendritic—branching or tree-like in form.
Diaphaneity—relative transparency. The diaphaneity of a mineral is
described as transparent, translucent, opaque, etc.
Dike—a tabular rock body, usually igneous in origin, which cuts across
the surrounding rock strata.
Dispersion—a measure of the ability of gemstones to separate complex
or white light into its component colors; often illustrated
with a prism. Gemstones that are capable of separating colors
of light widely are said to have high dispersion; gemstones
not so capable of separating white light into colors are said
to have low dispersion.
Dopping—the act of cementing a gemstone, either rough or partly
finished, to a dop-stick.
Dop-stick—the wooden stick or cylindrical piece of metal to which a
gemstone is cemented to facilitate handling during cutting and
polishing.
Dop-wax—the agent or cement used to secure a gemstone to a dop-stick.
Emerald cut—a rectangular or square faceted stone with beveled corners
whose surfaces are covered with several series of rectangular
facets.
Eocene—a division of geologic time, estimated to be the time from 50
to 40 million years ago; one of the older divisions of the
Cenozoic era.
Extrusive rock—igneous rock that has been extruded or forced out onto
the earth’s surface.
Facet—a single plane polished surface on a faceted gem.
Facet head—a device used in the cutting and polishing of faceted gems;
used to control the placement of facets and their relative
angles (fig. 7).
Facet table—the equipment used in the cutting and polishing of faceted
gems and the table on which most of the equipment is mounted
(fig. 7).
Feldspar—a group of closely related silicate minerals including
orthoclase, microcline, sanidine, plagioclase, labradorite,
and others.
Fire—the reflections of variously colored light from a precious opal;
also the different colors of light reflected from a faceted
gem owing to the dispersion of the mineral.
Fracture—the texture of a freshly broken surface other than a cleavage
surface, described as conchoidal, even, splintery, etc.
Gem—a cut and polished gemstone.
Gemology—the science dealing with the study of gemstones.
Gemstone—a mineral suitable for cutting into a gem; the term gemstones
is frequently used collectively to include both cut and
polished stones and rough stones.
Geode—a rounded or spherical rock cavity; commonly lined with
crystals.
Girdle—the portion of a faceted gem separating the crown from the
pavilion; the girdle may or may not be polished and usually
contains about 2 percent of the total depth of the gem (fig.
6).
Gneiss—a coarse-grained metamorphic rock having segregations of
granular and platy minerals that give it a more or less banded
appearance without well-developed schistosity.
Grain (pearl grain)—a unit of weight equal to 0.05 gram or 0.25 carat;
not the same as the Troy grain.
Granite—a granular igneous rock composed mostly of quartz, feldspar,
and commonly mica and/or hornblende.
Hexagonal—having six angles and six sides; a crystal system in which
the crystal faces are referred to four intersecting axes;
three of these axes are equal, lie in the same plane, and
intersect at angles of 60 degrees; the fourth axis is
perpendicular to the other three.
Igneous rock—rock formed by solidification from a hot melt.
Index of refraction—a measure of the relative ability of a gemstone to
“bend” incident light rays; sine of the angle of incidence of
a light ray divided by the sine of the angle of refraction.
Intrusive rock—rock that has been pushed (usually in a molten state)
among pre-existing rock strata, commonly along faults or
fissures. Intrusive rocks do not reach the earth’s surface but
are commonly exposed at the surface by later erosion.
Isometric—a crystal system in which the crystal faces are referred to
three equal intersecting axes at right angles to each other.
Lap—a disc-shaped piece of metal or other material which is
impregnated with diamond dust, or some other cutting or
polishing agent, that is revolved while the gemstone is worked
against it.
Lap plate—a metal plate to which a cutting or polishing lap is
attached, usually by means of a threaded bolt and wing nut.
The lap plate is attached to the shaft which is turned by the
motor under the facet table.
Lapidary—one who practices the lapidary arts; a gem cutter.
Limestone—a sedimentary rock composed mostly of calcium carbonate.
Luster—the appearance of the freshly broken or unweathered surface of
a mineral in reflected light (p. 5).
Main facet—as applied to the standard American brilliant cut, one of
the first eight facets cut on either the crown or pavilion of
a gem (fig. 6).
Matrix—the material in which a specific mineral is embedded; also the
rock to which one end of a crystal is attached.
Metamorphic rock—rock that has been changed from its original state by
heat, pressure, chemical action, or some combination of these
factors.
Millimeter—¹/₁₀ centimeter; approximately ¹/₂₅ inch.
Mineralogy—the science concerned with the study of minerals, including
their occurrence, composition, forms, properties, and
structure.
Monoclinic—a crystal system in which the crystal faces are described
in relation to three intersecting unequal axes, two of which
are at right angles and the third inclined.
Oligocene—a division of geologic time, estimated to be the time from
40 to 28 million years ago; part of the Cenozoic era.
Opaque—does not transmit light.
Orbicular—containing orbs or spherical or eye-like markings or
structures.
Orthorhombic—a crystal system in which crystal faces are referred to
three unequal intersecting axes at right angles.
Pavilion—the portion of a faceted gem below the girdle (fig. 6).
Pegmatite—a body of coarse-grained intrusive igneous rock, commonly
lens or dike shaped.
Perthitic—a plaid-patterned structure resulting from intermixture of
soda- and potash-rich feldspars.
Phantom crystal—a crystal outline seen within another crystal, mostly
due to entrapping of inclusions during the crystal’s growth.
Pleochroism—the property of transmitting different colors of light in
different crystallographic directions.
Point—a unit of weight equal to ¹/₁₀₀ (0.01) carat.
Porous—containing pores or void spaces.
Precambrian—a division of geologic time, estimated to be all of
geologic time prior to 550 million years ago; the time before
the Paleozoic era.
Preform—a gemstone that has been ground to a rough outline of the
finished shape of a gem.
Rhyolite—a fine-grained extrusive or shallow intrusive igneous rock of
approximately the same composition as granite.
Rough—uncut, not worked by a lapidary, not cut and polished.
Schist—a metamorphic rock that contains an abundance of oriented platy
minerals that enable the rock to be split with relative ease
parallel to the flat surfaces of the platy minerals.
Silicified—replaced by or containing a large amount of quartz or
silica.
Skill facet—a term often used for the pavilion girdle facets of the
standard American brilliant cut (fig. 6).
Specific gravity—the weight in air divided by the loss of weight in
water at a given temperature, or the weight of an object in
air divided by the weight of an equal volume of water; also
called relative density; the most commonly used standard
temperature for this measurement is 4° C. or 39.2° F.
Star facet—one of the eight facets surrounding the table facet of a
standard American brilliant cut (fig. 6).
Step cut—a mode of faceting in which the surface of the gem is covered
by a series of square or rectangular facets; stones thusly cut
are usually square, rectangular, or irregular with straight
sides in outline.
Streak—the color of a mineral when finely powdered; usually determined
by rubbing the mineral against a piece of unglazed porcelain.
Symmetry—the number, location, and balanced arrangement of crystal
faces in reference to the crystallographic axes or other
crystallographic planes or directions.
Synthetic gem—a gemstone manufactured by man that has approximately
the same chemical composition and properties as a natural
gemstone.
Table facet—the large horizontal facet found on the crown of many
gems, often called simply the table (fig. 6).
Tenacity—the resistance of minerals to breakage, described by such
terms as malleable, ductile, sectile, and brittle (p. 6).
Termination—the end of a crystal that is completely enclosed by
crystal faces, the crystal end that is not attached to the
matrix.
Tertiary—a division of geologic time, estimated to be the time from 60
to 1 million years ago; the Tertiary includes the Paleocene,
Eocene, Oligocene, Miocene, and Pliocene epochs (from oldest
to youngest).
Tetragonal—having four angles; a crystal system in which the crystal
faces are referred to three axes at right angles to each
other, two of which are equal and the third longer or shorter.
Translucent—allowing the passage of light but diffusing it
sufficiently so that objects on the other side cannot be
clearly distinguished.
Transparent—clear, allowing free passage of light so that objects on
the other side can be readily distinguished; opposite of
opaque.
Triclinic—a crystal system in which the crystal faces are referred to
three unequal axes, none of which are at right angles.
Tumbling—a process of polishing irregularly shaped gemstones (p. 17).
Vein—a tabular, irregular, or twisting mineral deposit that is thin in
relation to its length and breadth, usually the result of
solution or hydrothermal activity.
Vitreous—having luster, general appearance, or physical properties
similar to glass.
Vug—an unfilled rock cavity, commonly lined with crystals; may later
become filled by minerals owing to solution or hydrothermal
activity.