Chalcedony is quartz made of crystals far too small to see, packed together into a smooth, waxy, translucent mass. It is the parent material for a large family of gems, including agate, carnelian, chrysoprase, onyx and bloodstone. All of them share quartz's chemistry and hardness but look nothing like clear rock crystal. Chalcedony is tough, takes a fine polish and absorbs dye readily, which explains its long history in carving and its wide range of colors in shops today.
What chalcedony is
Quartz comes in two broad forms. Macrocrystalline quartz grows as visible crystals: rock crystal, amethyst, citrine and smoky quartz. Microcrystalline (or cryptocrystalline) quartz is made of tiny grains or fibers. Chalcedony is the fibrous kind. Under high magnification it is a dense tangle of quartz fibers, often mixed with a small amount of a related silica mineral called moganite, plus trapped water.
That structure gives chalcedony its typical properties:
- A slightly lower refractive index and specific gravity than crystal quartz, because of the pores and water between the fibers.
- A waxy to glassy luster and a soft, glowing translucency.
- A smooth, curved fracture with no cleavage.
- Very good toughness, since the interlocking fibers resist chipping.
- Porosity, which lets it soak up dyes.
Chalcedony usually forms at low temperatures from silica-rich water. It fills cavities in volcanic rock, lines cracks and veins, and replaces other materials such as wood or shell. Petrified wood, covered on the page about wood and petrified wood in jewelry, is often chalcedony that took the place of the original plant tissue.
Varieties of chalcedony
The trade uses dozens of variety names, based mostly on color and pattern. These are the most important.
Carnelian and sard
Carnelian is translucent orange to red chalcedony colored by iron oxide. Sard is a darker, browner red. The boundary between them is loose. Both were favorites for seals and intaglios in the ancient world.
Chrysoprase
Chrysoprase is apple-green to mint-green chalcedony colored by nickel. Fine, evenly colored, translucent material is the most costly chalcedony of all. Queensland in Australia has been the main source of top-quality chrysoprase since the 1960s, and historic deposits in Silesia (now Poland) supplied European jewelers for centuries. Green chalcedony dyed with chromium salts imitates it.
Agate and onyx
Agate is chalcedony with distinct bands, often curved and concentric. Onyx has straight, parallel bands, and sardonyx has bands of sard or carnelian with white. Black onyx in commercial jewelry is nearly always dyed chalcedony.
Bloodstone
Bloodstone, also called heliotrope, is dark green opaque chalcedony with red spots of iron oxide. Medieval Christian legend held that the red spots came from drops of Christ's blood, and carvers used it for religious scenes. India is the classic source. Bloodstone is a traditional March birthstone, listed on birthstones by month.
Blue chalcedony
Natural blue chalcedony ranges from pale gray-blue to lavender-blue. The blue usually comes from the way light scatters within the fine structure, not from a coloring element. Namibia, Malawi and Turkey have produced good material, and pale blue lace agate from Namibia is a banded form. Bright, saturated blue chalcedony is usually dyed. Rare, intense blue-green "gem silica" is chalcedony colored by chrysocolla, a copper mineral.
Other names
Plain white, gray or colorless chalcedony is often sold simply as chalcedony. Moss agate and dendritic agate are chalcedony with plant-like inclusions. Mtorolite is a green chromium-bearing chalcedony from Zimbabwe. Opaque, impurity-rich microcrystalline quartz is usually classed as jasper, though some mineralogists class jasper as a grainy quartz rock instead.
Porosity and dyeing
Chalcedony's pores make it one of the easiest gem materials to dye. The tiny spaces between fibers draw in liquids, and different layers absorb different amounts, so dye often brings out hidden banding.
Some common practices:
- Black onyx is made by soaking chalcedony in sugar solution and then treating it with sulfuric acid, which leaves carbon inside the stone.
- Red and orange "carnelian" is often pale agate soaked in iron salts and heated.
- Green chalcedony dyed with chromium or nickel compounds imitates chrysoprase.
- Bright blue, pink and purple chalcedony and agate are dyed with organic dyes or metal salts.
These methods are old and widely accepted, but they should be disclosed. Dyes from metal salts tend to be stable. Some organic dyes fade in sunlight. Signs of dye include colors that look unnatural, color concentrated along bands or cracks, and color that comes off on a cotton swab with acetone. Testing a stone this way can damage it, so leave it to a professional. Our page on gemstone treatments has a broader overview.
Uses through history
Chalcedony's toughness and fine texture made it a favorite of carvers. Ancient engravers cut seals, intaglios and cameos from it, especially layered onyx and sardonyx. Roman, Renaissance and later carvers made cups and vessels from large pieces. The town of Idar-Oberstein in Germany grew into a stone-cutting center largely on agate and chalcedony.
Today chalcedony appears as cabochons, beads, carvings, cameos and slices, and fine translucent chrysoprase and blue chalcedony are sometimes cut with simple faceted or carved surfaces.
Identifying chalcedony
A refractometer gives a spot reading of about 1.53 to 1.54. The specific gravity is lower than that of crystal quartz. Under magnification, chalcedony looks smooth and fine-grained with no visible crystals. Glass imitations may show gas bubbles or swirl lines, and plastic is lighter and warmer to the touch. The techniques are described on how to identify gemstones.
Durability and care
Chalcedony is about 6.5 to 7 on the Mohs scale and tough, so it suits rings, bracelets and everyday wear. Clean it with warm soapy water and a soft brush. Avoid harsh chemicals and solvents on dyed stones, and keep dyed or chrysoprase pieces out of strong sunlight for long periods, since some colors can fade. Chrysoprase can also lose water and fade if exposed to heat or left in very dry conditions for a long time.
Frequently asked questions
Is chalcedony the same as quartz?
Chalcedony is a form of quartz. It has the same chemistry, silicon dioxide, but it is made of microscopic fibers instead of large visible crystals.
What is the most expensive type of chalcedony?
Fine chrysoprase, with even, saturated green color and good translucency, is usually the most expensive chalcedony. Natural gem silica colored by chrysocolla is also costly.
Is blue chalcedony dyed?
Soft gray-blue to lavender chalcedony is usually natural. Bright, saturated blue chalcedony is usually dyed.
Can chalcedony be worn every day?
Yes. Chalcedony is close to 7 on the Mohs scale and is tough, so it handles daily wear well. Dyed stones need a little more care with chemicals and sunlight.