Chalcedony

Chalcedony is the form of Quartz that is compact and microcrystalline. It occurs in many different forms, colors, and patterns, and many varieties have been used as gemstones since antiquity. In the gemstone trade, the term Chalcedony is often used specifically to describe the white, gray, or blue translucent type of Chalcedony, but its technical term includes all additional varieties.

Chemical Formula SiO2
Color White, Blue, Red, Green, Yellow, Orange, Brown, Pink, Purple, Gray, Black, Banded, Multicolored
Hardness 6.5 – 7
Crystal System Hexagonal
Refractive Index 1.54 – 1.55
SG 2.63 – 2.65
Transparency Translucent to opaque
Double Refraction .009
Luster Vitreous to waxy
Cleavage None
Mineral Class Quartz (Chalcedony)
ALL ABOUT

Chalcedony is found in all types of colors, and its varieties are named based on color. The variety Agate is a multicolored form, well known for its distinctive banding patterns. Some forms of Chalcedony, such as Chrysoprase, are intensely colored, while others are light. In blue Chalcedony, the deeper the color, the more desirable the gemstone. The classic Chalcedony varieties are described below:

Agate

Multicolored banded variety, and the most well-known and unique form of Chalcedony. Agate itself has many different known varieties. All Agates contain distinctive patterns, which make each Agate different from the next. See the Agate gemstone page for more details.

Aventurine

Opaque, compact form, most often green in color, containing small Mica, Hematite, or Goethite scales which cause a glistening effect. See the Aventurine gemstone page for more details.

Bloodstone

Dark green to greenish blue variety speckled with red or brown spots. See the Bloodstone gemstone page for more details.

Carnelian

Red to amber-red translucent variety. Some forms of Carnelian are heat treated to darken the lighter tones. See the Carnelian gemstone page for more details.

Chrysoprase

Apple green variety; one of the more popular gemstone forms of Chalcedony. See the Chrysoprase gemstone page for more details.

Jasper

Opaque form of brown, often multicolored, spotted, or speckled. See the Jasper gemstone page for more details.

Onyx

May describe solid black Chalcedony, banded black and white Chalcedony (usually in parallel banding), or any Chalcedony with a black base and white upper layer. See the Onyx gemstone page for more details.

Sard and Sardonyx

Sard is the brownish to brownish-red, transparent to translucent variety. The related Sardonyx has parallel bands of brownish to red alternating with white or sometimes black bands.

Tiger’s Eye

Pseudomorph of compact Quartz after the fibrous mineral Crocidolite. Tiger’s Eye is famous for its chatoyant effect and glistening sheen. See the Tiger’s Eye gemstone page for more details.

USES

Chalcedony is the source of a host of different gemstones. Most gem forms are polished as cabochons and beads for use in bracelets, necklaces, earrings, and as costume jewelery. They are also used as animal carvings and cameos. Besides for all the named varieties that are used as gems, the translucent gray and blue forms of Chalcedony have recently gained much popularity.

VARIETIES

Varieties specific to Agate, Jasper, Onyx, and Tiger’s Eye are listed separately.

Agate – Banded variety of Chalcedony. For individual varieties, see the Agate mineral page.
Aventurine – Opaque, compact Quartz / Chalcedony containing small Mica, Hematite, or Goethite scales which cause a glistening effect. Aventurine is most often green but may also be other colors such as gray, orange, and brown.
Binghamite – Chalcedony with dense, parallel inclusions of Goethite. Its color is a multicolored yellowish to reddish, and it exhibits chatoyancy. Binghamite was found in the Cuyuna Iron Range in Minnesota, and is used as a rare gemstone.
Bloodstone – Dark green to greenish blue variety of Chalcedony speckled with red or brown spots.
Carnelian – Red to amber-red translucent variety of Chalcedony.
Chrysoprase – Apple green variety of Chalcedony.
Fire Agate – Form of Agate or Chalcedony that is iridescent with a play of colors or “fire” similar to that of Opal. Fire Agates usually have botryoidal growth included in their interior. The play of color is caused by inclusions of Goethite or Limonite.
Holly Blue – Translucent, violet-blue gemstone variety of Chalcedony.
Jasper – Opaque form of Chalcedony, most often used to describe brown, yellow, or reddish colors. For individual Jasper varieties, visit the Jasper mineral page.
Moss Agate – Chalcedony containing dense inclusions of green Hornblende that cause the pattern to resemble moss. Moss Agate is technically not a true Agate as it lacks the banding patterns of Agate.
Onyx – Type of Chalcedony gemstone whose meaning can have several connotations. Its most practical gemstone definition describes a solid black Chalcedony, or a banded or layered black and white Chalcedony.
Plasma – Dark green variety of Chalcedony. It often contains small white or yellow spots.
Prase – Light to emerald green, transparent to translucent Quartz / Chalcedony, with coloring caused from inclusions of green minerals, such as Actinolite, Hedenbergite, Chlorite, or Malachite.
Sard – Brownish to brownish-red, transparent to translucent form of Chalcedony. A gemstone can sometime borderline between falling under the definition of Sard and Carnelian, and either term used will be valid.
Sardonyx – Form of Agate with parallel bands of brownish to red alternating with white or sometimes black bands.
Tiger’s Eye – Pseudomorph of compact Quartz/Chalcedony after the fibrous mineral Crocidolite. Tiger’s Eye is famous for its chatoyant effect.Chalcedony is sometimes dyed to create more vibrant colors. This is especially true of vivid blue, red, and pink colors. White Chalcedony is also occasionally dyed light blue to resemble natural blue Chalcedony.Chalcedony is abundant and found worldwide, with individual sources too numerous to mention.Most of the Chalcedony gemstones are unique in color and style, and can be easily distinguished from most other gemstones. Some forms of Opal may resemble Chalcedony, but Opal lacks the hardness of Chalcedony.
Chrysoberyl is a hard, tough, and durable gem. Although lacking the fire of other gemstones, Chrysoberyl in its various forms can be quite valuable. Most Chrysoberyl gems are yellow, though some are brown, green, or orange. Chrysoberyl is best known for its important varieties Alexandrite and Cat’s Eye. Alexandrite is a rare and expensive form that exhibits different colors in natural and artificial light. Cat’s Eye is Chrysoberyl that is polished into a cabochon and highly chatoyant, displaying a sharp line of light through the center of the stone.
SIMILAR GEMSTONES
Chalcedony SOURCES
Chalcedony TREATMENTS AND ENHANCEMENTS

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