Chalcedony used in Jewelry
Source: Occuring worldwide
Chalcedony is a fiberous, translucent to opaque, compact microcrystalline variety of quartz that occurs in white, gray, and grayish-blue hues. Chalcedony is formed by silica-rich water percolating through cavities and fissures in volcanic rock. Chalcedony is a relatively porous material that is easily dyed to alter or enhance the color. Chalcedony that has distinct banding is called "agate." The name "chalcedony" (calcŽdoine French, chalzedon, Kalzedon German, Piedra de luna Spanish or Italian) is derived from the ancient sea port of Khalkedon, at the crossroads of Europe and Asia in Constantinople (now Istanbul, Turkey).
Pure chalcedony is white, but there are several varieties of chalcedony that go by different names depending on coloration. Green chalcedony gets its color from trace amounts of nickel oxide, and goes by the name 'chrysoprase,' while red varieties of chalcedony goes by the name 'carnelian' or 'sard.' Black chalcedony is referred to as onyx, and brown chalcedony is referred to as jasper. Banded varieties are referred to as agate.
Chalcedony Earrings by Michael Sherman
Chalcedony Agate Geode
Both agate and chalcedony are typically cut into a cabochon or beads. Chalcedonies are found worldwide. Chalcedony was perhaps one of the first materials used by early man to form tools and arrowheads, due to its durability and abundance, and flint used as a fire-starting tool, is a variety of chalcedony.
Chalcedony is crystallographically identical to the quartz, and belongs to the tectosilicate family of minerals. Chalcedony forms in the hexagonal/trigonal crystal system with a microcrystalline crystal habit.
Chalcedony (Microcrystalline Quartz) Chemistry & Physical Properties
Chalcedony (Microcrystalline Quartz) Optical Properties
Bloodstone (also used for Hematite) is a bluish (bluestone) to greenish variety of chalcedony that is flecked with spots of red color from the presence of iron oxide. The red spots can take on the appearance of splattered blood, thought to be the blood of Christ during the Middle Ages. Bloodstone occurs in Australia, Brazil, China, India, and the United States.
Bibliography and Reference on Chalcedony
1. Judith Crowe, The Jeweler's Directory of Gemstones . DK Publishing.
2. A.C. Akhavan, Twinning in Quartz Crystals www.quartzpage.de
3. GIA, Characteristics of Citrine, Amethyst & Smoky Quartz www.gia.edu
4. Renee Newman, Gemstone Buying Guide . International Jewelry Publications; 2nd edition
5. Antoinette L . Matlins, Antonio C. Bonanno, Gem Identification Made Easy . Gemstone Press