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" (Calcédoine 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 red chalcedony is referred to as Jasper.
Chalcedony Earrings by Michael Sherman
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.
Agate
Most agates occur as nodules in ancient volcanic rocks or lava where cavities where filled by silicate and zeolitic minerals deposited in layers upon the walls of the cavity, with the first deposit forming the outer skin of the agate. When the deposition process does not proceed to the point of filling the cavity, a hollow space is left causing a geode (above, right). The interior of the geode can be encrusted with amethyst or quartz crystal. Banding found in Plume Agate, Sagenite, Thunderegg, and Tube Agate is caused by silicate (green) iron oxide or limonite (red), and is referred to as banded agate, riband agate or striped agate.
Dendritic agates (above, left) and moss agates (above, right) have delicate fern-like patterns that are formed from the presence of iron and manganese ions. Mexican agate or "cyclops agate" has an array of colors including black, brown, gold, green, and red embedded in chalcedony.
Chalcedony is crystallographically identical to the quartz. Chalcedony has a hardness of 7 on the Mohs scale. The toughness of Chalcedony is Good. Chalcedony belongs to the Hexagonal or Trigonal crystal system with a Microcrystalline crystal habit. Specific gravity is between 2.70. Chalcedony has a refractive index of 1.550, and a dull to waxy luster.
Carnelian & Sard
Carnelian (aka Cornelian or sard) is an translucent to opaque orange-red, brownish-red, pure red, or pale yellow variety of chalcedony. Carnelian was used in ornamental objects and jewelry throughout ancient Egypt, Persia, and Sumeria as early as 3000 BC. The name "sard" is derived from one of the places that the stone was found, near the ancient Roman city Sardius (below, right), in the kingdom of Lydia, now in the Manisa province of of modern day Turkey. During Roman times, Carnelian was a popular stone for use in cameos, intaglios, and signet rings. The German village of Idar-Oberstein grew around the mining and cutting of agate and carnelian in the 1400s.
Bernd Munsteiner cut Carnelian, Michael Zobel setting
Iron oxide and/or hematite impurities cause carnelian's reddish color. Carnelian is found in Brazil, China, India, Colombia, Germany, Scotland, the United States, and Uruguay. Some stones that are marketed as Carnelian can actually be dyed chalcedony.
Bloodstone
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.
Chemical composition: SiO2
Semi-Precious Stones