Table of Contents
This interesting list may lead to a more careful watch for precious and semi-precious stones and their names. Most gems are crystals of unusual quality in regard to transparency and color; they are to be looked for in those rocks that have crystallized slowly, or in cavities that have been slowly filled by the growth of crystals. Pegmatite dikes are therefore apt to yield gems. Vugs and rounded masses that may be vugs should be carefully examined for gem crystals. The weight of gems is given in carats. A carat is 205 milligrams, or 3 1/6 grains, very nearly. One ounce Troy equals about 151¾ carats. It should always be remembered that crystals of unusual beauty and perfection may have considerable value as mineral specimens.
Diamonds
Diamonds are found in peridotite and serpentine, and in the soft material formed by the weathering of these rocks. In Brazil, they are found associated with a peculiar flexible sandstone, itacolumite. As peridotite and serpentine are plentiful in the Precambrian of Canada, diamonds should be looked for in these rocks. In the productive diamond fields of South Africa, the gems are found in weathered rock in volcanic necks. Diamond placers occur in South Africa, India, and Brazil; and small diamonds have been found in California placers. Diamonds of merchantable size and in considerable quantity have been taken out of weathered peridotite near Murfreesboro, Arkansas. As diamonds of considerable size have been found in the glacial drift of some of the Northern States, there is a chance of finding a diamond field in Ontario, from which much of the drift was derived. In panning for gold, diamonds should be watched for. They can be recognized by their rounded form, the curved faces of the crystals, and their extreme hardness. Diamond scratches quartz easily; quartz scratches glass; therefore the ability to scratch glass is not a test for diamond.
Rubies and Sapphires
Rubies and sapphires are corundum crystals of red and blue color, respectively. They may be found in syenite, nepheline-syenite, anorthosite, peridotite, serpentine, and other basic igneous rocks, particularly when these rocks are pegmatite. In New York and New Jersey, rubies have been found in crystalline limestone that is banded with gneiss. In North Carolina, these gems have been found in chlorite, mica, and talc, in a dike of olivine and serpentine. In Burmah, long the producer of the finest rubies, they are found in crystalline limestone and gneiss, but are mostly obtained from placers formed by the weathering of these rocks, Sapphires are found in a peculiar lamprophyre in Montana, and also in andesite in the same state.
Gem corundum crystals, on account of their great hardness and freedom from weathering, have survived the wear and tear of the years; they should be watched for in panning for gold. They can be recognized by their hexagonal form, and they scratch quartz easily. Corundum of a blue color has been found in the Ontario corundum deposits, but the crystals are not of gem quality.
Emerald
Emerald is a green gem variety of beryl, found in pegmatite dikes, in calcite veins in carbonaceous crystalline limestone, and sometimes in slate and mica schist, Aquamarme is a blue-green variety, Beryl crystals look like crystals of apatite, but are very much harder (H = 7.5 to 8). Its specific gravity is 2.7. Valuable beryl crystals have been obtained as a by-product in mining muscovite mica.
Spinel
Spinel is a compound of magnesia and alumina, MgO.Al2O3. Some varieties contain iron or chromium. The common color of gem spinel is ruby-red, but other colors may give value to good transparent crystals; H = 8; G = 4 The crystals are in the cubic system, usually octahedrons. Ruby spinel is sometimes found with the true ruby, the crystals of which are hexagonal. Spinel is found in crystalline limestone, serpentine, peridotite, etc. Fine, blue spinel crystals have been found in Quebec.
Diaspore
Diaspore, Al2O3.H2O, is hydrated alumina. The common color is white; crystals of a yellowish color, like topaz, are found; also some of a brownish tint; H = 6.5 to 7; G = 3.4; Crystals, in the orthorhombic system, shaped a good deal like those of topaz; but topaz is harder than quartz, while diaspore is a little softer or equal to quartz in hardness. It is found with corundum.
Chrysoberyl
Chrysoberyl is oxide of beryllium and aluminum; it is of considerable value when in clear, golden-yellow crystals. The crystals are orthorhombic; H = 8.5; G = 3.5 to 3.8. Jewelers call the yellow variety chrysolite. The green variety, alexandrite, is valued as a gem. Cat’s eye is green chrysoberyl showing a peculiar play of colors, the chatoyant effect. A variety of quartz that is also called cat’s eye owes its name to the same peculiarity. Chrysoberyl is found with tourmaline, garnet, and beryl.
Phenacite
Phenacite is similar to beryl in composition, silicate of beryllium, while beryl has aluminum in addition. Phenacite is found in colorless crystals, which are among the most brilliant gem stones known; H = 8; G = 3. It looks a good deal like quartz but is harder, and the crystals are of a different shape, roughly like the cleavage pieces of calcite. Phenacite is found with beryl, emerald, amazonstone, and topaz, in granite pegmatite.
Peridot or Olivine
Peridot is the jeweler’s name for olivine. Clear transparent olive-green crystals are of value as gems. H = 6.5 to 7; G = 3.5. It is found in the basic igneous rocks and in certain metamorphic rocks, such as impure dolomite.
Topaz
Topaz is used as a gem when the crystals are clear and perfect. The prevailing color is honey-yellow. It is found in vugs in granite pegmatite, rhyolite, and other acid rocks, also in the surrounding schist, etc. Its hardness (H = 8) and brilliancy make it of some value as a gem. The crystals are in the orthorhombic system, and are apt to be long with a pointed end. A broken end shows good cleavage across the length. The finest crystals sell for as much as $100 each.
Garnet
Garnet crystals are sometimes perfect and clear enough to be of value as gems. Most of the varieties of garnet are used as gems when transparent crystals are found. Jewelers use red and yellowish brown garnets under the name hyacinth. Garnet crystals can be recognized by their roundish shape and hardness (H = 6.5 to 7.5). Garnet is a rather heavy mineral (G = 3.15 to 4.3). Good crystals may be found, in schists, particularly mica schist and chlorite schist, in slate, in crystalline limestone and serpentine near contacts with igneous rocks, in pegmatite, and in gneiss. Beautiful crystals are found in mica schist along the Stickeen River, B. C. Green chrome-garnet of gem quality has been found in Orford township, Que., in vugs in crystalline limestone.
Very fine gem garnets have sold for as much as $50 each. Good crystals of one carat weight have been valued at from $1 to $5 each.
Tourmaline
Tourmaline, when in fine pink, red, or green, crystals, is of value; H = 7 to 7.5; G = 3. Gem tourmaline may be found in pegmatite dikes, and at the contact of intrusive granite with such metamorphic rocks as schists, gneiss, and crystalline limestone. Clear, finely colored crystals may be of considerable value. Transparent tourmaline crystals are dichroic, that is, the color is different when the crystals are viewed lengthwise and across the length. Crystals of brown tourmaline of gem quality have been found in crystalline limestone in Ross, Ontario, and at Calumet Falls, Clarendon, and Hunterstown, Quebec.
Hyacinth or Jacinth
Hyacinth or jacinth, is zircon, of a rich red color. Orange and brown crystals when transparent may be of value. Zircon crystals can be recognized by their square build and their hardness (H = 7.5). A colorless variety may be mistaken for diamond, but it is heavier (G = 4.7) and not so hard. Crystals suitable for gems may be found in pegmatite dikes, and at contacts of igneous intrusives with metamorphic rocks, particularly crystalline limestone and nepheline-syenite. Fine zircon crystals are found in Sebastopol and Brudenell Townships, Renfrew County, Ontario.
Turquoise
Turquoise is hydrated phosphate of aluminum and copper, of a sky-blue, green, or greenish gray color, and a waxy luster; it is translucent or opaque; H = 5 to 6; G = 2.6 to 2.9. It is brittle. It is found in veins and in irregular masses in acid and intermediate volcanic rocks that are more or less altered. In Persia, it is found of gem quality in brecciated acid volcanic rocks and in slate in contact with them. In New Mexico, it is found in very much decomposed volcanic tuff. Turquoise is apt to change color, but Persian turquoise retains its color indefinitely.
Quartz
The varieties of quartz used as gems and ornamental stones. Rock crystal, amethyst, cairngorm or smoky quartz, sagenite or fleche-d’amour (quartz crystals with needle-like crystals of rutile shot through them) are some of the varieties of quartz crystals used as gems. They are found in pegmatite dikes and other acid rocks. They are to be looked for in vugs (geodes) in these rocks. Cavities in sandstone and quartzite have yielded very fine crystals. Opal is silica combined with water. Precious opal shows a beautiful play of colors. Fire opal is red to yellow with a gleam like fire when it is turned. Opal is found filling cavities and seams in trachyte, porphyry, basalt, etc. Agate is found mostly as rounded masses in basalt and serpentine. Where these rocks have weathered and broken down extensively, the hard, rounded masses of agate, and other similar varieties of quartz, are found on the shores of lakes and rivers. Jasper, particularly when red and banded with other colors, is a fine ornamental stone.
Various Precious Stones
Jade is the name given to varieties of pyroxene and hornblende, of a green color, valued as an ornamental stone.
Spodumene belongs to the pyroxene group; it is a silicate of aluminum and lithium. The variety, hiddenite contains a little chromium which perhaps gives it the green color for which it is valued; it resembles emerald, but is yellowish green, while the emerald is pure green. When first put on the market about 1880, a 2½-carat stone cut from hiddenite sold for $500, but the values have decreased considerably since that time. Another member of the pyroxene group, diopside, has been found in fine emerald-green crystals, which are of value as gems.
Smaragdite, is a bright emerald-green variety of hornblende; is considered of value as an ornamental stone.
Rhodonite, a silicate of manganese, is sometimes found as pink or rose-reddish crystals, which have some value as precious stones. Large masses of rhodonite have been found in Russia, where it is used as a gem and also as a decorative stone.
Crocidolite, a silicate of iron, magnesium, calcium, and sodium, is a kind of asbestos of a blue color. By oxidation and addition of silica, it has sometimes been changed into a hard stone of a bright yellow or brown color, with the peculiar luster called chatoyant; it is known as tiger-eye. Its fibrous structure gives it the look of petrified wood.
Epidote sometimes occurs in transparent green crystals of gem quality.
Moonstone is the name given to varieties of feldspar, albite, and orthoclase, which have the chatoyant play of colors, usually very delicate blue and green shades.
Sunstone or aventurine, is a variety of feldspar showing peculiar fiery reflections, probably due to small crystals of hematite, etc. A variety of quartz showing this effect is also called aventurine.
Amazonstone is a feldspar, labradorite, which is valued as an ornamental stone because of its fine, blue chatoyant effect. It is found in large quantities and of good quality in Labrador. Another feldspar, microcline, is sometimes chatoyant and of a beautiful green color; the amazonstone, of Pike’s Peak, Colorado, is of this kind.
Perthite is a mixture of two feldspars, orthoclase and albite, laminated in such a way as to give the internal reflections of light that are characteristic of aventurine. The color of the mineral found at Perth, Ontario, the first discovery, is flesh-red.
Chlorastrolite is found as pebbles on the shores of Isle Royale, and on the North Shore of Lake Superior. They have been weathered out of the amygdaloid basalt (trap) of those regions. It is of a mottled opaque-green color and hard enough to take a good polish. It is sometimes chatoyant. It is hydrated silicate of aluminum, calcium, etc.
Thomsonite, a hydrated silicate of alumina, lime, and soda, is found in the Lake Superior amygdaloid basalt (trap), and as rolled pebbles on the beaches. It owes its value to its concentric layers of flesh-red, white, yellow, and green.
Fluorspar, when in fine colors and in large masses, is used as material for vases and other ornamental objects; also as decorative stone.
Serpentine of rich green color is used for ornamental purposes. When found in large enough quantity, it may be used as a decorative building stone.
Lazulite is a hydrated phosphate of alumina and soda. It is of a fine, opaque blue color, like that of lapis lazuli.
Sphene or titanite is prized as a gem when found in transparent crystals. Sebastopol and Brudenell Townships, Renfrew Co., Ont., are localities famous for sphene crystals. The crystals are mostly opaque and not of gem quality.