CHALCEDONY

Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite.

Color: It exhibits a wide range of colors, including white, gray, blue, green, red, pink, and black, which can be uniform or arranged in bands.

Chrysocolla chalcedony is not found in many locations globally, making it a rare gemstone.

Varieties: Includes a variety of stones, including carnelian, prase, plasma, onyx, and chrysoprase.

Usage: Widely used in jewelry, carvings, and as a decorative stone.

Formation: It forms from watery silica gels at relatively low temperatures, often in association with other microcrystalline quartz varieties like agate.

It can be found in various geological settings, including volcanic and sedimentary rocks, where it precipitates from moving solutions.
Chalcedony can also occur as a pseudomorph, replacing other materials like organic matter in petrified wood or coral.
Note: In the gemstone trade, “chalcedony” sometimes specifically refers to white, gray, or blue translucent chalcedony, but its technical term includes all varieties.

High Demand:
Its vibrant color and rarity have led to it being highly valued in the gem trade, especially for well-cut, saturated blue stones.

Other relatively rare chalcedony colors:

Pink: is one of the rarer varieties and is a heavily desired material by craftsmen across the globe. The Crystal Collector™ found several beautiful specimens in Arizona. Please watch the video.

Red: especially a rich, blood-red color, is uncommon. Most red chalcedony exhibits a pale pinkish-orange hue, rather than a true ruby red.

Blue: is also relatively rare, with sapphire-blue varieties from Oregon being particularly sought after.

Green: Chrysoprase, a green variety of chalcedony colored by chromium, is also considered rare.

What is another name for chalcedony?

Chalcedony is a catchall term used for translucent polycrystalline quartz. Different coloured varieties have different names (eg, carnelian, chrysoprase, jasper). Where no variety name is in general use, the name chalcedony is used prefixed by the color.

USES

It is used for a variety of purposes, including jewelry, decorative items, and in spiritual practices due to its purported healing and calming properties. It’s also been used historically for tools, weapons, and as a pigment for cave paintings.

MOHS HARDNESS

It has a Mohs hardness rating of 6.5 to 7. This means it is relatively hard and can be scratched by a diamond, but it will also scratch a piece of glass. 

LUSTER

It has a waxy luster, and may be semitransparent or translucent. It can assume a wide range of colors.

CHEMICAL COMPOUND

Its chemical compound is SiO2 (silicon dioxide)

WHERE TO FIND

It can be found in various locations worldwide, both in the United States and internationally. It’s commonly found in sedimentary or volcanic environments, sometimes in geodes, or as a petrifying agent in fossils.

Where is chalcedony found in the US?

South Dakota: The State’s best known chalcedony is its colorful and beautiful Fairburn agates. Named after a community near a very prolific agate deposit in Custer County, these brightly colored banded agates are similar to Lake Superior agates found in Michigan and Dryhead agates from Montana.

Alaska: Agates, jaspers, and petrified wood.

California: Including Death Valley and Nipomo, with various colored chalcedony.

Arizona: Petrified Forest National Monument.

South Dakota: Fairburn agates.

Nebraska: Blue Chalcedony (Blue Agate) is the state stone.

Other locations: Montana, Wyoming, Nevada, Oregon, and other states with specific varieties like agate, chrysocolla, bloodstone, and blue chalcedony.

Internationally:

Turkey: The name “chalcedony” originates from the ancient city of Chalcedon, in present-day Turkey.

Greenland: White to greyish chalcedony in volcanic strata, and a light blue variety.

Brazil: Known for agate varieties.

India: A source of gem-quality chalcedony.

Madagascar: A source of gem-quality chalcedony.

Other locations: Burma, Mexico, Namibia, and Russia.

DIAGNOSTIC PROPERTIES

It is a cryptocrystalline form of silica (SiO2) with diagnostic properties that include a waxy luster, a hardness of 6.5 to 7 on the Mohs scale, and a variety of colors due to inclusions. It is typically semitransparent to opaque, with a refractive index of 1.535-1.539 and a density of 2.55-2.91 g/cm³. It also exhibits a conchoidal or sub-conchoidal fracture and a white streak.

Cleavage: None observed.
Toxicity: Generally considered non-toxic.
Fluorescence: Some varieties may fluoresce a weak green or blue under ultraviolet light, especially those containing uranium.
Inclusions: It often contains inclusions of other minerals, which can affect its color and appearance.