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. (See Chalcedony: http://www.suite101.com/article.cfm/gems...). 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
