oxides
Chalcedony
SiO2
- Also known as
- chalcedony quartz
- Crystal system
- trigonal
- Hardness
- 6.5–7 on the Mohs scale
- Lustre
- waxy
- Colour
- White, grey, blue, or almost anything when banded or stained
- Where it forms
- Precipitates from silica-rich water at low temperature, lining and filling cavities in volcanic and sedimentary rock. It grows as microscopic fibres rather than visible crystals.
- Chiefly from
- BrazilUruguayIndiaTurkeyBotswana
Why it is that colour
Usually trace iron, nickel or chromium. The characteristic milky translucency is not a colour at all but light scattering from the microscopic fibre structure.
Safety
No hazard from handling. As with all quartz, cutting produces silica dust.
Where it comes from
Common and mined worldwide. Much of the blue and purple chalcedony on sale is dyed, a practice going back to Roman times and industrialised in Germany in the nineteenth century.
Historical use
One of the most important materials in the ancient world, because it is hard, takes fine detail and does not cleave. Nearly all classical engraved gems and cylinder seals are chalcedony in one form or another.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
What is attributed to it, and since when
Named for Chalcedon in Asia Minor and used throughout the ancient Mediterranean for seals and amulets. Pliny discusses it. Specific modern meanings attached to individual colours are much later.
Described, not endorsed. Dating a claim is not the same as assessing it — but an association that first appears in a book published in 1985 is not an ancient inheritance, whatever the label says. A sincere account of feeling better is compatible with the stone doing nothing — see the placebo effect.
Worth knowing
Chalcedony is the umbrella under which a great many trade names sit. Agate, carnelian, chrysoprase, onyx, jasper, bloodstone, sard, plasma and several others are all chalcedony distinguished by colour, banding or opacity. None of them is a separate mineral species and all of them are quartz.
For a long time chalcedony was thought to be pure microcrystalline quartz. It is now known to contain a proportion of moganite, a distinct silica mineral with a different structure, which gradually converts to quartz over geological time. Older chalcedony contains less of it — so the ratio works, roughly, as a clock.
Identity checked against IMA-CNMNC master list, June 2025.