Volume 12, pages 383-395, 1927
THE NOMENCLATURE OF SILICA
GILBERT HART, Birmingham, Alabama.
In its many forms, silica has been used in all stages
of civilization, from the ancient flints of the Stone Age to the modern silica
laboratory ware. Because of its many uses, and of the many varied forms in which
it occurs, silica has been called by more names than any other mineral. Many of
the older names of flint are now so
obsolete that repetition is needless, but many of the present-day names for
quartz gems are unknown save to a few jewellers. Then, too, the exact research
of the modern laboratory has shown several distinct crystallographic varieties
of silica; some of which are closely connected with the temperatures experienced
in their life-history.
The many different names, and their different
connotations, which are now in use for silica minerals, call for a
classification and arrangement in a more ample, yet more concise manner than is
to be found in the usual discussion of the varieties of silica. This article is
written with the hope of making a scientific classification of these names, so
that the use of the different terms will no longer be a cause for tedious
searching for definitions.
I. CRYSTALLOGRAPHIC VARIETIES
These varieties are named in the order formed with
Recrystallization changes occur at the temperatures noted when ample time is
allowed for the action, often in the laboratory only in the presence of
catalysts. Besides the changes at these critical temperatures, there are
probably similar changes from unstable forms towards quartz at atmospheric
temperatures, especially after long time intervals. With fairly rapid cooling or
heating intermediate forms may not occur in their stable zone, but a direct
change from one to another without the intermediate product may take place. Most
of the recrystallization changes noted are found to occur at both ascending and
(A) SILICA GLASS - amorphous, a true non-crystalline glass, stable below the
melting point and above the "gc" temperature.
Quartz Glass, Fused Silica, Fused Quartz, are other names for this supercooled
liquid. In most forms at atmospheric temperatures there are traces of
(B) CRISTOBALITE - isometric, or pseudo-isometric, "gc" range is at
1710° where Cristobalite changes to glass as temperatures rise, or glass to
cristobalite as they fall.
Christobalite, an alternate spelling.
Beta Cristobalite, also called High Cristobalite, is the high temperature
product, forming in the "gc" range in cooling. It is isometric, and in
cooling recrystallizes to
Alpha Cristobalite, or Low Cristobalite, at 200-275°, providing cooling through
the "ct" and "tq" ranges has been too rapid for
recrystallization. It is tetragonal.
(C) TRIDYMITE - hexagonal, bipyramidal. "ct" range is at 1470°, where
cristobalite changes to tridymite on cooling. Glass may crystallize as tridymite
at 1670° if the cooling was too rapid through the "gc" range.
Beta Second Tridymite, or Upper High Tridymite, is the high temperature product,
forming in the "ct" range in cooling, and which recrystallizes to
Beta First Tridymite, also called Lower High Tridymite, at 163° if cooling was
too rapid for the "tq" transformation. This in turn alters to
Alpha Tridymite, or Low Tridymite, at 117°, which is the usual tridymite of
Asmanite - a meteoric tridymite, related to the above series.
Vestan - a doubtful silica mineral, probably to be ascribed to tridymite.
Granuline - a doubtful pulverescent mineral which seems allied to tridymite on
(D) QUARTZ - hexagonal, forms from tridymite in the "tq" range at 870°
in cooling. Glass may change to crystalline quartz at about 1400° providing
cooling was too rapid for the "gc", "gt" and "ct"
Beta Quartz, or High Quartz, is the high temperature product, forming at the
"tq" point. It is hemihedral. On cooling it recrystallizes to Alpha
Quartz, also called Low Quartz, at 573°, yielding the stable low temperature
mineral. It is tetartohedral, showing polarity along the c axis and is divisable
Right Hand Quartz and
Left Hand Quartz
(E) CHALCEDONY - a cryptocrystalline, or very finely fibrous mineral, which has
not been successfully located in the thermal equilibrium diagram. Heating to
725-850° usually results in an alteration to tridymite, which thereafter acts
as normal tridymite. Chalcedony is usually found as a deposit from solutions,
and may be a mixture of glass and quartz, or more probably an intermediate
product in the dehydration of the opal colloid. Various subdivisions of
chalcedony have been made on optical grounds.
Chalcedony - biaxial, positive, elongation positive.
Chalcedonite - biaxial, negative.
Lussatite - biaxial, positive, parallel elongation.
Quartzine - biaxial, positive, negative elongation
Jenzschite - differently soluble, but of same S. G. as chalcedony.
Melanophlogite - possibly impure chalcedony.
Sulfuricin - probably a chalcedony rich in sulphur.
(F) COLLOIDAL SILICA - is usually hydrous, and is commonly described under opal.
alpha and beta, right hand and left hand quartz are present in the varieties
here considered. Rarely, perhaps, are included tridymites or cristobalites, but
their occurrence is rare, and specimens are not often found under other than
their type names.
(A) CLEAR LARGE CRYSTALS, which may or may not have crystal faces, but are
essentially large single individuals.
(1) Colorless, transparent, lustrous.
(a) Crystal - indicative of the clearness of ice.
(b) "Diamond" indicative of the clearness of true diamond, and of the
use as a substitute for diamond. Quartz "diamonds" are usually of
local value only, though sometimes sold as imperfect diamonds. The locality of
the specimen is shown in its name.
Brazil or Brazilian Diamond
Cape May Diamond
Hot Springs Diamond
Isle of Wight Diamond
Lake George Diamond
(c) "Pebble" or "Stone" indicative of the water-worn
surface of crystal clear
Rain Stone - a double meaning of a water-worn pebble supposed to represent
Vellum or Vallum Stone.
(d) Other clear quartzes:
Beryl - old name, applied particularly to engraved stones, only very rarely used
with this meaning at the present time.
Dragonite )Water - worn quartz with brilliant luster, supposed to be
Dragon's Eye )petrified eyes of the mythological dragon.
Jewel of Perfection - Japanese term for rock crystal.
(2) Colored crystals, color usually uniform, but may be zoned or irregular in a single individual.
(a) Violet, purple:
Amethyst - typical name for this color.
Occidental Amethyst - differentiates true amethyst (quartz) from other minerals
of similar color.
Oriental Amethyst is applied to exceptionally beautiful specimens of amethyst.
Siberian Amethyst refers to a dark colored amethyst whose color by artificial
light is fine red.
(b) Blue, indigo:
Sapphire - usually applied to corundum gems, and when used in connection with
quartz usually has that name added.
Water Sapphire -- usually applied to cordierite gems, but also to quartz,
(c) Yellow, golden:
Citrine is the typical name for this color, and includes all quartz of yellow
"Topaz" - much of the topaz of commerce is yellow quartz, or
decolorized smoky quartz.
d) Smoky browns, smoky yellows:
Smoky Quartz is typical of any smoke-like color.
Cairngorm - the Scotch name for particularly pellucid smoky quartzes, which is
now applied to most which are suited for gems.
)alternate spellings adopted in different localities.
Scotch Pebble - this term is also applied to small agates freed from the lavas,
and worn by water to rough polish.
(e) Color red:
Apricotine - yellowish red.
Jacinto - dark red.
Mont Blanc Ruby.
"Ruby" applied as is diamond to quartz of the color of true ruby, but
always with a qualifying name to show origin. This use of ruby and diamond is
quite distinct from that of topaz, which is adopted by jewellers as a name for
(f) Color black:
Morion - deep black, often almost opaque, but more usually will transmit light
fairly well, and almost totally reflect angular light.
VARIETIES NAMED FROM PECULIARITIES OF CRYSTALLIZATION OR SHAPE.
Most are of the rock-crystal variety, but the other types occur both colored and
(1) Parallel groupings and intergrowths of large individuals.
Babel or Babbel Quartz - rock-crystal with flat pyramidal growths on the large
pyramidal faces, the tiers of which have a fanciful resemblance to the tower of
Cavernous Quartz - with deep etched cavities parallel to the faces.
Sceptre Quartz - parallel grouping of small knob-shaped crystal atop a slender
(2) Fibrous groupings.
Barrel Quartz - corrugated veinlets, whose sheaves of fibres are barrel shaped.
Cross-course Spar - radiated vein-quartz.
(3) Other groups.
Drusy Quartz - small crystals in parallel growth, as crusts, or lining geodes,
or in central part of veins.
Globular Quartz - porphyritic quartz phenocrysts in spherical outline, may be
twins showing as spherical sectors or round individuals.
Mineral Blossom - drusy quartz.
Potato Stone - quartz geode.
Twisted Quartz - simple quartz prism warped as through pressure and now made up
of spirally arranged individuals.
VARIETIES NAMED FROM INCLUSIONS OF FOREIGN MINERALS. Sometimes in definite
crystals, irregularly dispersed or arranged in adherence to crystallographic
lines or planes; also inclusions of liquid or gas.
(1) Crystalline inclusions.
(a) Spangles: Aventurine is the common type name, and includes all spangled
Avanturine is an alternate spelling.
Gold Quartz contains native gold in visible spangles.
Gold Stone - yellow iron oxides, simulate gold.
Hyacinth of Compostella - red hematite inclusions.
Imperial jade - green aventurine.
Imperial Yu Stone - green actinolite (?) inclusions.
Lizote - blue inclusions of silver ore.
Rusty Quartz - discolored by iron oxides.
Rubasse or Rubace - red hematite inclusions. This name is also applied to quartz
stained red by artificial means.
Sinople or Sinopal - red hematite spangles.
Sunstone - very rare variety with yellow spangles.
Sagenite is the type name, and rutile is the most common acicular mineral in
Byssolite, fine greenish actinolite or asbestus needles.
Cupid's Darts - goethite inclusions.
Fleches d' Amour - rutile needles.
Hair Stone - crowded full of a matted mass of acicular crystals.
Hedgehog Stone - radiated needles of geothite.
Love Arrows - rutile needles.
Onegite - goethite inclusions.
Reticulated Quartz - rutile needles in rectangular patterns.
Rutilated Quartz - rutile needles.
Thetis Hair Stone - green acicular actinolite inclusions.
Venus Hair Stone.
(c) Fibres, usually parallel and yielding a cat's eye effect when cut across
fibres; also sometimes showing asterism.
Cat's eye is the type name.
Asteria shows asterism.
"Crocidolite" applied to a replacement of the original crocidolite by
quartz which retains enough of the silicate to color the replacement and often
to give cat's eye effects.
Hawk Eye or Hawk's Eye - applied to crocidolite replacement.
Hungarian Cat's Eye.
Occidental Cat's Eye.
Quartz Cat's Eye.
Sapphire Quartz - blue because of crocidolite, often only faintly chatoyant,
indicating an almost complete replacement of the fibrous crocidolite.
South African Cat's Eye - crocidolite replacement.
Star Quartz - ) asteriated quartz.
Star Stone )
Tiger's Eye - brownish to yellow crocidolite replacement, showing alteration
previous to the introduction of silica.
Wolf's Eye Stone.
(d) Layers of clay or scaly mineral deposited on former crystal planes, which
were covered by later deposits of the same orientation on the quartz
Capped Quartz, in which the shells of quartz are separable.
Ghost Quartz, in which the outline of the smaller crystal is visible.
Phantom Quartz, in which chlorite grains show the smaller crystal.
Skeletal Quartz, in which the smaller crystal is not of the same form as the
(e) Densely distributed inclusions, usually this type is found in rocks of which
silica is the major part; and the inclusions represent the residual of other
minerals included in the original sediments.
Micaceous Quartz, etc.
Liquid inclusions, usually of water, also carbon dioxide, or hydrocarbons,
visible through the presence of a bubble of air or other gas which moves as the
specimen is turned.
Gaseous inclusions, often in films in cracks yielding an iridescent stone or
more rarely in larger masses, showing when opened as pungent odors
Cotterite - has a metallic pearly luster and probably belongs here.
Eldoradoite - an iridescent quartz.
Iris - often natural, but may also be artificially produced.
Stink Quartz - yields odor of sulphur dioxide when struck a sharp blow with the
ARTIFICIALLY ALTERED QUARTZ, especially changed in color by gem craftsmen to
provide a more profitable sale for off-colored stones. While such stones are not
natural minerals, they are sold often as such, and their classification here is
Brazilian Topaz - smoky quartz altered to yellow by heat.
Burnt Amethyst - purple amethyst changed to brownish yellow by heat; much
cairngorm and citrine are also so altered.
Harlequin Stone - artifically colored crocidolite.
Orange Topaz - smoky quartz changed to yellow by heat.
Rubasse - quartz stained red; this name is also applied to a red aventurine.
Spanish-Topaz - smoky quartz altered to yellow by heat.
Note also that much "Topaz" is quartz which has been made of an
acceptable yellow hue by heat treatment.
MASSIVE AND MICROCYSTALLINE QUARTZ
It can be definitely resolved by the microscope into quartz,
sometimes in heterogeneous macroscopic crystals, often very finely fibrous.
(A) MASSIVE QUARTZ, small individuals with no systematic orientation.
Amethystine Quartz - of amethyst color.
Citrine Quartz - color of citrine.
Ancona Ruby - red massive quartz.
Brazilian Ruby - rose colored.
Ferruginous Quartz - red, brown or yellow massive quartz, colors due to hematite
Rose Quartz - massive rose-red to pink, may have an opalescent luster.
Greasy Quartz - milky white, with greasy luster.
Hyaline - opalescent white quartz.
Milky Quartz - milky white, vitreous luster.
MICROCRYSTALLINE, usually fibrous, not resolvable into its components except
with the microscope.
Jasper - is the type name, and varieties are named from color or
Basanite - velvety black.
Lydian Stone )
compact and close-grained.
Test Stone )
Touch Stone )
Aztec Stone - usually refers to green calamine, but also applied to green
Chalchihuitl = Aztec Stone.
Mother-of- Emerald = Prase.
Prase - translucent green quartz, usually spotted.
Blood Stone )
Blood jasper ) massive dark green jasper with red spots often
Ezteri - similar to bloodstone, but with red veinings instead of spots.
Creolite - banded or mottled in shades of brown.
Egyptian jasper ) banded in yellow, red, brown, and black.
Egyptian Pebble )
Jasperine - a banded jasper.
Morlop - mottled jasper.
Moss Jasper - opaque to translucent, with dendrites.
Ruin jasper - irregular bands resembling ruins.
Striped jasper - broad bands of color.
QUARTZ Rocks. Most quartz rocks can be ascribed to (A) or (B) of this class but
are separated on account of their geological importance. The size of individuals is usually dependent on origin.
(1) Clastic Rocks:
Silex - a trade name for quartz sand.
Glass Sand - an especially pure quartz sand.
Granular Quartz .
(2) Organic Siliceous Rocks:
(3) Chemically deposited, concretionary, etc.
Night Mare Stone
Milk Stone, white flint pebble
Witch Riding Stone
Sycite, fig-shaped flint pebble
Geyserite, usually opaline, rarely anhydrous.
(4) Vein Quartz, usually of igneous origin.
CRYPTOCRYSTALLINE VARIETIES OF SILICA
Chalcedony is the type name, but this has been divided into several optically
distinct types, as noted above. Usually very finely fibrous and sub-microscopic.
Surface usually botryoidal, fracture hackly, luster waxy, translucent to opaque.
(A) VARIETIES NAMED FROM COLOR.
(I) Color uniform:
Cacholong - whitish cloudy.
Chalcedony is usually whitish, and is often used to refer to any uniformly
California Moonstone - white to gray.
Moonstone - usually applied to feldspar, also to white chalcedony.
Mother Stone - whitish chalcedony.
Occidental Chalcedony - somewhat opaque, whitish.
Oriental Chalcedony - very translucent white chalcedony.
Rainbow Chalcedony - structurally in thin concentric layers, but of uniform
color, may exhibit iridescence when cut across layers.
White Agate - uniform white chalcedony.
Mohava Moonstone - translucent lilac-hued chalcedony.
Violite - purple.
Blue Chrysoprase - blue.
Blue Moonstone - blue.
Keystoneite - blue.
Sapphirine - blue chalcedony, note also that this is a silicate mineral, and is
also applied to blue quartz.
Zafirina - blue chalcedony.
Chrysoprase - green translucent chalcedony.
Jade - true jade is a silicate, but the term is often wrongly applied to green
Plasma - green translucent chalcedony.
Canary Stone - rare yellow carnelian.
Cambay (or Camboy) Stone - carnelian.
Carnelian - translucent red chalcedony.
Sard - rich brown translucent chalcedony.
Banded, color and structure, Agate is the type name, and refers to any banded
(a) Straight bands:
Onyx - is typical of straight bands one of whose colors is white.
Carnelonyx - white and red bands.
Carnelian Onyx - white with red bands.
Chalcedony Onyx - white and pale colored bands.
Chalcedonyx - bands of gray and white.
Nicolo - black or brown base, with bluish white top band.
Oriental Sardonyx - black base, white intermediate band and brown or red top
Sardagate - white and orange-red bands, may be semi-transparent.
Sardonyx - white and brown bands.
Saturnine Onyx - with very dark lower band, giving the stone a dark appearance
Amber Agate - yellowish, translucent.
Amberine - yellowish green.
Blood Agate - red to pink.
Carnelian Agate - with predominating bands of carnelian.
Cer Agate - chrome-yellow.
Oriental Agate - finely marked and very translucent.
Riband Agate - parallel bands.
Sardachate - with predominating bands of red carnelian.
Semi-carnelian - yellow agate.
Striped Agate - wide parallel stripes.
Curving bands, often concentric, probably formed by successive layers deposited
in spheroidal cavities, as in fauns.
Eye Agate - concentric rings, usually showing a dark center.
Cyclops - a single large eye.
Ring Agate - concentric differently colored bands, often with pale chalcedonic
center, or a druse.
Rainbow Agate - shows iridescence when cut across the concentric structure.
Broken bands, zigzag, or otherwise discontinuous.
Brecciated Agate - angular fragments of agate cemented by amethystine quartz.
Fortification Agate - parallel zigzag lines, as though an agate broken and
cemented by very narrow bands of chalcedonic silica.
Ruin Agate - zigzag bands resembling ruins.
Colors mottled, perhaps due at times to inclusions but more often there is no
discontinuity of the silica, merely a changing of pigment.
Catalinite - green, red, and brown mottlings.
Cloudy (or Clouded) Agate.
Cloudy (or Clouded) Chalcedony.
Frost Stone - gray ground, with scattered patches of white.
Prismatic Moonstone - cloudy chalcedony.
Rice Stone - a ground color spotted with white spots resembling rice
Sandy Sard - brown chalcedony spotted with darker browns.
St. Stephen Stone - with round blood-red spots.
White Carnelian - cloudy white or very pale reddish.
VARIETIES DUE TO LUSTER.
Wax Agate - yellow agate with pronounced waxy luster.
VARIETIES NAMED BECAUSE OF MECHANICAL INCLUSIONS. Some of the uniformly colored
chalcedonies belong here because their color is due to some definite mineral, as
the blues are often due to chrysocolla, but their name is applied to the color
and not the impurity. Many impurities are dendritic.
Moss Agate is the type name for dendritic chalcedony.
Fancy Agate - with particularly delicate markings.
Flower Stone - when the dendrites are flower-like.
Scenic Agate - when the dendrites suggest landscapes.
Tree Agate - dendrites resemble trees.
Other solid inclusions:
Myrickite - bright red cinnabar inclusions.
Opal Agate - alternating layers of opal and chalcedony.
Other inclusions of liquid or gas. Perhaps the iridescence of Rainbow Chalcedony
and Agate are due to thin air films between the concentric layers.
- hollow nodules of chalcedony partly filled with water.
Water Agate - shell of chalcedony containing a bubble of water.
ARTIFICALLY ALTERED CHALCEDONY AND AGATE. This mineral is porous enough to
absorb dyes, and agate is often differentially porous so that different layers
will absorb different dyes, yielding a varicolored product. Colors are, like
quartz, altered by heat.
Burnt Carnelian )color made red by heating.
Burnt Stone )
Emeraldine - stained green.
False Lapis )stained blue.
Swiss Lapis )
MIXTURES AND INTERGROWTHS OF QUARTZ, JASPER AND CHALCEDONY
Some of these close associations of the varieties of silica suggest that quartz
is an ultimate product of recrystallization which may take place after very long
periods of time.
Agate jasper - intermediate between jasper and chalcedony, a close mixture,
often banded or veined.
Hemachate - light colored chalcedony spotted with red jasper.
Hyaline Quartz - quartz with bluish opalescent cast due to the presence of
Jaspagate - opaque jasper with chalcedonic inclusions.
Jasponyx - onyx, part of whose layers are jasper and part chalcedony.
Kinradite - jasper with spherulites of quartz.
Texas Agate - agate jasper.
(A) ORGANIC PSEUDOMORPHS. Many fossils are preserved by silicification of their
soft parts, or of their calcareous shells and bones. Particular names are given
SILICA REPLACING OTHER MINERALS, as fluorite, barite, etc., but - definite names
for such replacements are limited to
Haytorite - chalcedonic replacement of datolite.
ROCKS, AND OTHER MIXTURES PREDOMINATELY SILICEOUS
The rocks listed in III-(C) often contain appreciable impurities and also may
consist wholly of silica. Many other rocks contain large proportions of silica,
particularly the acid igneous rocks, of which possibly vein quartz is an
extreme. Most sedimentary rocks, except limestones and coal, contain large
amounts of detrital quartz. Most metamorphics also carry large percentages of
free silica, both as recrystallized material from the sediments, and as material
added by the metamorphic agencies. Furthermore, quartz is an important part of
many weathering products, alluviums, gossans, etc.
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