The Complete Chalcedony Guide

Some of the most common types of chalcedony are:
Bloodstone: a dark green chalcedony with deep red spots
Agate: curved or angular-banded varieties of chalcedony
Chrysoprase: translucent to semi translucent, light to medium yellowish green
Onyx: larger slabs of straight parallel layers of different colors

Chalcedony is a gem composed of cryptocrystalline (microscopic) quartz crystals that form in tightly interlocking, fibrous masses. All chalcedonys are semi-translucent to opaque but rarely will eye-clean cut stones be cut form it.

Chalcedonies may be porous, so care must be taken not to immerse the gem in substances that could change its appearance. As with most gems, a damp, soft cloth, or scrubbing with a soft-bristle toothbrush, are the best ways to clean these two forms of quartz.

Dyeing: chalcedony can be dyed in practically all colours, due to its porous nature. Much of the black chalcedony sold on the market is dyed black. In banded material, some bands may be dyed while others remain white.
Heating: some yellow to brown material may be heat treated to result in redder colours
.

Chalcedonies may be porous, so care must be taken not to immerse the gem in substances that could change its appearance
Use damp, soft cloth, or scrubbing with a soft-bristle toothbrush
Chalcedonies possess an extraordinary toughness, allowing them to be carved in large flat cabochons, intaglios, cameos, tablets or even plates or bowls. Idar-Oberstein, twin cities in Germany, have been known for developing the cutting and carving industry for this gem since the 1700s.

Chalcedony exhibiting unusual banding or landscape scenes are particularly prized, as are agates containing dramatic dendritic inclusions. Iris agates and fire agates exhibiting strong iridescent colours are also collected. Finally, carved chalcedony, such as intaglios or cameos, may be collectible if they have provenance that can be identified as typical of a certain era. Signed gems from a known lapidary artist are also collectible.