CHALCEDONY, a form of quartz encompassing all cryptocrystalline varieties

Chalcedony is the generic name for "cryptocrystalline quartz", meaning the individual crystals are too small to see. The term is frequently used when the specific variety is uncertain (for example, a sample that would be called an agate if it showed banding), or in technical references.

The primary varieties of chalcedony are as follows:

• Agate is a banded variety (sometimes with translucent bands)
• Bloodstone is green with red speckles
• Carnelian is yellow to orange
• Chrysoprase is green
• Flint is generally black with a fibrous microscopic structure
• Jasper is any colorful agate
• Onyx is black, white, or alternating black and white
• Sard is yellow to brown
• Sardonyx is banded, alternating sard and (usually white) onyx