Chalcedony
Chalcedony is a cryptocrystalline form of quartz made of microscopic fibrous crystals too small to resolve without magnification, chemically identical to ordinary quartz but structurally distinct from it. It serves as the parent material for a large family of banded and colored varieties, including agate, onyx, carnelian, and chrysoprase. Chalcedony typically forms a waxy, translucent to semi-translucent mass rather than distinct visible crystals.
Chalcedony properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | SiO2 |
| Mohs hardness | 6.5–7 |
| Crystal system | Trigonal (cryptocrystalline) |
| Luster | Waxy to vitreous |
| Streak | White |
| Cleavage | None |
| Fracture | Conchoidal |
| Specific gravity | 2.6–2.64 |
| Transparency | Translucent |
| Common colors | Blue-gray, White, Brown, Various (often dyed) |
| Rarity | common |
Geology & background
Chalcedony precipitates from silica-bearing groundwater, most often filling cavities and fractures in volcanic rock, where it can line the walls of gas bubbles left behind in cooling lava to eventually form nodules and geodes. It also replaces organic material during fossilization, producing silicified wood, and fills veins in a variety of host rocks. Classic localities include the agate beaches of Lake Superior, the thunderegg fields of Oregon, and the vast agate deposits of Brazil and Uruguay.
Rockhounds prize chalcedony for its durability, workability, and the enormous range of colors and banding patterns produced by trace impurities and growth stages, from the concentric rings of agate to the moss-like inclusions of moss agate. Because it takes a good polish and resists scratching well, chalcedony has been carved into seals, cabochons, and beads since antiquity. Collectors should note that many commercial agates are dyed to enhance or alter their natural coloring.
How to identify Chalcedony
Chalcedony shows a waxy to dull luster, is translucent to nearly opaque, and ranges through white, gray, blue, brown, and countless dyed colors, with a white streak. Hardness is 6.5 to 7, matching coarser quartz, and it breaks with a smooth conchoidal fracture rather than showing any cleavage. Banded, botryoidal, or nodular habits are typical. Chalcedony is often confused with jasper, which is simply an opaque, more heavily included variety of the same cryptocrystalline quartz family.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Chalcedony is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Chalcedony worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Agate, onyx, carnelian, and chrysoprase are all varieties of the same underlying mineral material, chalcedony.
- Chalcedony's fibrous microstructure, invisible without a microscope, sets it apart from the coarser interlocking crystals of ordinary quartz.
- Many so-called black onyx specimens sold commercially are actually gray agate that has been artificially dyed.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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