Chalcedony is a catch-all term for cryptocrystalline quartzes (quartz with microscopically small crystals). The chalcedony family as a whole includes agate, jasper, carnelian, chrysoprase, onyx, bloodstone, aventurine, flint, chert and sard. As a marketing term in the gemstone industry, however, "chalcedony" refers more specifically to semitransparent or translucent chalcedony with a solid color (commonly pale bluish-gray) and nearly waxlike luster. The color can also be white, blue, purple, pink, yellow, orange or red (but not orange-red, as that stone is known as carnelian). Blue and purple are the most popular hues.
Some are unsure how to pronounce "chalcedony" - the most common form is kal-SEH-doh-nee; also accepted are chal-SEH-doh-nee, KAL-seh-DOH-nee and CHAL-seh-DOH-nee. The name probably comes from Chalcedon or (Calchedon), an ancient port city on the Sea of Marmara in modern-day Turkey. Good blue and purple chalcedony are still being mined in Turkey and several locations in Africa, including hard-to-get Namibian material. Indonesia is producing purple chalcedony along with other colors, including yellow to deep orange. The U.S.A. also boasts active blue and purple chalcedony mines.
Chalcedonies can be either transparent or translucent, solid or patterned. The most common forms are agate and jasper. Agate is usually translucent and has clearly defined bands and markings, while jasper is generally opaque, more irregular and less defined. Chalcedony forms in rounded crusts, rinds or stalactites in volcanic and sedimentary rocks. The stone is extremely porous, so it takes dye easily and is frequently enhanced. Chalcedony has a slightly lower density and hardness than other quartz. Protect it from scratches, sharp blows, harsh chemicals and extreme temperatures. Another unique characteristic: chalcedony can be a pseudomorph, the atom-by-atom replacement of one mineral for another without changing the original mineral's external appearance. Petrified palm wood is a well-known example. In the Petrified Forest National Monument of Arizona, the remains of an entire ancient forest was transformed into chalcedony. Dinosaur bone is another chalcedony pseudomorph. The process is nature's preservation of an organic substance in its original appearance.