Gemology 101: Introduction to Gemology
Article: Jasper Gemstones
The minerals grouped as jasper belong to the quartz family. Quartz is one of the most abundant minerals on Earth. It's formed primarily from silica and oxygen, but includes trace amounts of dozens of other minerals. Because of these mineralogical impurities, quartz is found in myriad forms.
Mineralogists have divided the quartz family into two main groups: crystalline and cryptocrystalline. Cryptocrystalline is also called microcrystalline quartz. Due to its distinct crystal formation, crystalline quartz is usually transparent. Rock crystal, amethyst, citrine, and rose quartz are well-known examples of crystalline quartz.
The atoms in cryptocrystalline quartz pack together to form stones that are either opaque or translucent. The atoms take the form of fibers rather than crystals, and the stones often contain water or air trapped between the layers of fibers.
The cryptocrystalline quartzes are informally divided into two groups: jasper and chalcedony (kal-ced-nee). The jaspers are often opaque, while the chalcedonies tend to be transparent. Most mineralogists don't distinguish between the two, simply referring to both as cryptocrystalline.
Because there are too many cryptocrystalline quartzes to describe in one article, we'll use the commonly accepted groupings. The chalcedony family generally includes agate, aventurine, bloodstone, carnelian, chrysocolla and chrysoprase. Onyx, prase, sard, sardonyx and tiger's eye are usually classified as jaspers.
Although not now regarded as precious stones, the chalcedonies and jaspers were very highly valued in ancient times, not only for their beauty, but also for their reputed magical and medicinal properties.
Picture, or scenic, jasper gets its name from the pictures of scenes, animals or objects formed by the patterns in the stone. The unique markings are caused by fossilized algae trapped within the stone.
Ancient peoples used jasper to protect against pain, especially from childbirth.
Onyx is a catchall term used to describe several stones. The pure black stone often marketed as onyx is dyed chalcedony. The onyx used in architecture isn't a form of quartz at all, but rather belongs to the calcite family. Onyx agate is distinguished by parallel stripes. It's often used for cameos, and it's the mineral described in this article.
The stripes, or bands, on onyx are usually alternating black and white. When properly cut, the concentric bands look like eyes, leading most ancient peoples to use onyx to ward off the evil.
Although generally regarded as a protective stone, onyx gained a reputation as a stone of discord. It was never worn by lovers, nor was it worn at night, as it provoked nightmares. Prase, which means leek in Greek, is a pale green form of jasper. Prase is often included in the opal family, as one of the common opals. (Common opals lack the flash and fire precious opals are noted for. There's also a green stone recently discovered in Australia that has been named Imperial prase by its discoverers. They chose that name because it resembles chrysoprase. Their choice of name may or may not be recognized by mineralogists.
Sard is a reddish-orangish brown stone that was often paired with onyx by the ancients in the belief that the positive effects of red would dispel the negative aspects of the black onyx.
Sardonyx is very similar to onyx in that it has parallel stripes, and similar to sard in that the stripes are alternating reddish-brown and white. People wore sardonyx next to their hearts to heal depression. It was also thought to improve communication.
Tiger's eye starts life as crocidolite, an asbestos-like mineral. Over time, quartz replaces the crocidolite fibers, creating a silky-looking yellowish-brown gemstone. Tiger's eyes get their name from their chatoyancy. Chatoyancy refers to the appearance of an eye moving across the stone as it is rotated. The jasper family also includes petrified dinosaur bone and petrified wood. In both these instances, the original organic material was replaced by quartz. (This process is also called fossilization.) The replacement substance retains the same shape of the animal or plant. By its very nature, petrified material is ancient.
Turritella is very similar to petrified dinosaur bones, in that it's snail shells in which the original organic material has been replaced by quartz. It's found primarily in the state of Wyoming.
All the jaspers are cut into cabochons (rounded shapes) rather than faceted. Most are also made into beads or carved into ornamental objects. Like the other members of the quartz family, jaspers are found worldwide.
By Sandra I. Smith, Writer
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