Chalcedony is a catch all term that includes many well known varieties of cryptocrystalline quartz gemstones. They are found in many locations, in many colors and color combinations, and in sedimentary, igneous, and metamorphic rocks. Chalcedony includes carnelian, sard, plasma, prase, bloodstone, onyx, sardonyx, chrysoprase, thundereggs, agate, flint, chert, jasper, petrified wood, and petrified dinosaur bone just to name a few of the better known varieties.
Because of its abundance, durability, and beauty, chalcedony was, except for sticks, animal skins, bones, plain rocks, and possibly obsidian, the earliest raw material used by humankind. The earliest recorded use of chalcedony was for projectile points, knives, tools, and containers such as cups and bowls. Early man made weapons and tools from many varieties of chalcedony including agate, agatized coral, flint, jasper, and petrified wood.
In the petrified forests of the Desert Southwest, many of the original tree tissues have been replaced by chalcedony in the formation of petrified wood. What remains is a mineral replica of the original tree in various fantastic colors. The brilliant reds and greens are caused primarily by traces of iron oxides. Some of the best displays of petrified wood can be seen in eastern Arizona’s Petrified Forest National Park.
In all ages many varieties of chalcedony have been used as gems, and many colored varieties are still cut and polished as ornamental stones. Unless it is intricately carved or featured, Chalcedony is valued much less than it once was. Commercial production is primarily in Uruguay, Brazil and southwestern Africa.
Chalcedony occurs in many varieties and colors. If chalcedony is conspicuously color-banded, it may be called agate and with other minerals it has various other names including:
Onyx: This striped, semiprecious variety of agate has black and white alternating bands. Onyx is used in carved cameos and intaglios because its layers can be cut to show a color contrast between the design and the background. Onyx is one of the 12 stones mentioned in the Bible as adorning the breastplate of the high priests.
Bloodstone: Also called Heliotrope, this dark-green variety of chalcedony has nodules of bright-red jasper distributed throughout. Bloodstone was greatly prized in the Middle Ages and was used in sculptures representing flagellation and martyrdom.
Chrysoprase: This brittle, translucent form of chalcedony owes its bright, apple-green color to the inclusion of nickel silicates, but heating or prolonged exposure to sunlight causes the color to fade.
Plasma: This semi-translucent, microgranular variety of chalcedony occurs in various shades of green, caused by amphibole or chlorite inclusions. Plasma often has nodules of gray quartz or Bloodstone throughout its mass. It has long been used for carvings and mosaics.