Varieties of Quartz
Rock crystal is the term applied to quartz when it is clear and colorless.
Milky quartz is the milky variety, the whiteness being due to imperfections in the crystallization, such as cracks, bubbles, etc.
[Smoky quartz] is the cloudy brown-colored variety, which results from the presence of small quantities of organic matter (hydrocarbons) in the quartz. If the color is so dark as to be almost black it is termed morion. In the above cases the color will disappear if the stone is heated. Pebbles of smoky quartz from Cairngorm, Scotland, have been so widely used as semiprecious stones that they have come to be known as cairngorms.
Citrine, or false topaz, is a clear yellow variety, the color again due to the presence of organic matter. It is distinguished from true topaz by the lesser hardness, this having the hardness of 7, while true topaz has a hardness of 8.
[Amethyst] is quartz with a violet color, due to the presence of small quantities of manganese. To be suitable for cutting into gems, the color must be deep or the small pieces will appear almost colorless. It is widely used today as a semiprecious stone in jewelry; and in the fifteenth century it had the traditional virtue of making the wearer sober-minded, whether he had taken too freely of wine, or was over excited by love-passion.
[Rose quartz] gets its pale-red color from the presence of a small amount of titanium. It is widely distributed, but is more abundant in the Black Hills of South Dakota.
Aventurine is quartz which has inclosed tiny scales of mica or hematite giving it a spangled appearance.
[Prase] is a green quartz, the color being due to the inclusion of fibrous crystals of green actinolite.
Cat’s Eye is a quartz which has inclosed silky fibers of asbestos. When this is cut parallel to the fibers, the effect is opalescent. The colors are greenish, yellowish-gray, and brown. This form, however, is not to be confused with the true or Oriental Cat’s Eye, which is chrysoberyl and has the hardness of 8.
[Chalcedony]
SiO₂
Non-crystalline, occurring in botryoidal, stalactitic or concretionary masses; hardness, 7; specific gravity, 2.65; color white when pure; luster waxy; translucent to transparent on thin edges.
In addition to the crystalline form, silica is freely deposited in an amorphous or cryptocrystalline form which has the same properties as quartz, except the crystal faces. This is called chalcedony, and it occurs in seams, cavities and free surfaces. When the surface of a chalcedony deposit is free it has a waxy luster. It is generally very brittle and breaks in a peculiar splintery manner. Like quartz it also has a great many varieties, according to the impurities present. Its wide distribution, hardness, and the manner in which it can be chipped have made this a most important stone in the history of the development of civilization. The early men first broke it into rough tools, such as knives, axes, spear points, etc., and used these as cutting tools, of one sort or another, because they held their edge better than most stones. We apply, to the people who used only these chipped stones as tools, the term “Men of the Old Stone Age,” or the period is termed the Palæolithic Age. Later men learned how to grind the edge to a smoother outline, and this much shorter period is termed the Neolithic Age. The use of flints for the first tools is world-wide, and the American Indian when discovered was still using chalcedony in its rough-hewn state.
“There the ancient Arrow-maker
Made his arrow heads of sandstone,
Arrow heads of chalcedony,
Arrow heads of flint and jasper,
Smoothed and sharpened at the edges,
Hard and polished, keen and costly.”
Chalcedony is the proper term to use when the color is white to translucent, in which case the surfaces are usually botryoidal and waxy.
[Carnelian] is chalcedony which is clear red in color and translucent. This is one of the first stones used for ornamental purposes and for engraving. Carnelians with figures engraved on them were used by the Egyptians, Assyrians and The Children of Israel, at least 2000 B.C.; and the Egyptian scarabs of the fifth or sixth century B.C., were often carved from this variety of chalcedony, as well as from jasper and agates.
The brownish varieties are termed sard.
[Chrysoprase] is an apple-green variety of chalcedony the color being due to the presence of nickel oxide. This is by no means as common as most of the varieties of chalcedony, and was long prized as a gem.
[Plasma] is chalcedony with a leek- to emerald-green color, and the same stone when it has small red spots of jasper in it is termed blood-stone, or heliotrope. These red spots are said by tradition to be drops of the blood of Christ.
[Jasper] is a deep red chalcedony, the color being due to hematite, which is so abundant as to make it opaque. A brown variety colored by limonite is also called jasper, and even green jaspers are found. In all cases the opaque character is common.
[Flint] is an impure brown chalcedony, usually forming concretions. The color is due to organic matter. Flint is mostly found in limestone or chalk, and the concretions are the result of the small particles of silica scattered through the rock being dissolved, and then reprecipitated about some organic center. Generally the silica was obtained by the dissolution of small fossils, like the shells of diatoms or sponge spicules.
[Hornstone] and [Chert] are simply impure varieties of flint, brown in color, and with a splintery fracture.
[Agate], [Plate 32], is a banded or cloudy chalcedony which has formed in a cavity, the layers of different color representing deposition from water, carrying first silica with one impurity, then later, silica with another impurity. Gradually the cavity has been thus filled with silica; and when the mass is freed by the weathering away of the surrounding rock, these banded masses are found. Sometimes the manner of deposition has changed, and while the outer part of the cavity was filled with chalcedony, the central part will contain quartz crystals. On account of the beauty of the colors, and the unusual way in which they may be developed, agates are widely used for semiprecious jewelry and objects of art, and this has been true since ancient times, the name itself coming from the River Achates in Sicily. The center for cutting and polishing agates is at Oberstein, Germany, where this work has been carried on since the middle of the fifteenth century. In spite of the many fine natural colors in agates, they are sometimes artificially colored, in many cases by methods which are kept as “trade secrets.” The color seldom penetrates far; so that even slight chipping reveals whether an inferior agate has been taken and colored up, or whether the stone is natural. Moss agates are chalcedony which has inclosed dendritic masses of some one of the manganese compounds as shown under manganite, [p. 73].
[Onyx] is a variety of agate where the bands are alternately black and white; while sardonyx is agate with red or brown bands alternating with the white. Such agates as these are especially desirable for cameo work, where the figure is carved in the chalcedony of one color, and the other color makes the background.
Silicified or agatized wood is a form of chalcedony, where silica has replaced wood, molecule by molecule; so that in good specimens, all the structure of the wood is still retained, and when thin sections are made it can be studied under the microscope almost as well as modern wood. This takes place under water, usually, if not always, in fresh water. Such fossilized wood is widely distributed in the western United States, the most famous cases being the Fossil Forest of Arizona, now a National Reservation, and the fossil trees in the Yellowstone National Park.
[Opal]
SiO₂·H₂O
[Pl. 33]
Non-crystalline, massive, stalactitic or nodular; hardness, 6; specific gravity 2; all colors; luster vitreous, resinous, or pearly; transparent on thin edges.
Opal differs from chalcedony in having water, usually about 10%, incorporated in its structure. This is water of crystallization, and not firmly held; so that, if opal is heated in a closed tube to above 100 C., it is given off as a vapor. Opal is distinguished from chalcedony by its lesser hardness, and the resinous to pearly luster. It forms in cavities, in layers often of extreme thinness.
Opal is originally the product of the dissolution of silicate minerals in hot acid waters, the resulting gelatinous silica, when it is deposited and hardened, becoming the opal. There are many varieties, some of them highly prized as gems in spite of the moderate hardness and opacity of the mineral. Gem-quality opal gets its opalescent character from the successive deposition of thin films of opal, the light penetrating and being reflected from different films. This breaks up the white light and causes the play of colors which is the charm of this gem.
Precious opal, in which the play of colors is finest, comes mostly from Hungary, Mexico, and Queensland. The opal was a favorite stone from before Roman times, and in its early history was a charm against the “evil eye.” During the nineteenth century for some reason it came to be considered an unlucky stone.
[Fire opal] is a hyacinth-red to honey-yellow variety, which has a fire-like play of color, and is found in Mexico and Honduras.
Common opal does not have the play of color, but comes in a variety of colors; is waxy or greasy in luster; and occurs mostly as fillings of seams or cavities, especially those in igneous rocks, like the steam holes in lavas, etc. It is found in Cornwall, Penn., in Colorado, California, etc.
Opal-agate is a variety in which there are color bands, and it is widely distributed.
Opalized wood is formed in exactly the same manner as agatized wood, much of the fossil wood called silicified being really opalized.
Siliceous sinter is the porous mass of opal which is so frequently deposited about hot springs and geysers. It is readily recognized by its porous character.
The shells of the diatoms, which are microscopic plants, are made of opal; and while they are so small, there is certainly no other plant so abundant or omnipresent, living as it does in every pool, lake, or sea by the millions. These shells are very indestructible so that they accumulate at the bottom of ponds, bogs, and sea-bottoms, making at times extensive deposits. This material in quantities is termed diatomaceous earth, or tripolite (from Tripoli where it was first used commercially). It is used as a polishing powder for metals, marble, glasses, etc.