Chalcedony (Agate, Jasper, Chert)
SiO₂ · Trigonal (microcrystalline) · also called agate, jasper, chert, flint, carnelian
Chalcedony is quartz built from crystals too small to see, which gives it a waxy lustre and a tough, even texture. It is the material behind agate, jasper, carnelian, flint, and chert — the same mineral wearing different names depending on colour and pattern. Because it breaks with predictable sharp edges, it was the dominant toolmaking stone of prehistory.
Properties
- Mohs hardness
- 6.5–7
- Streak
- white
- Lustre
- waxy, vitreous, dull
- Colour
- Any colour. Banded material is called agate; opaque single-colour material is jasper; grey-brown sedimentary masses are chert or flint.
- Cleavage
- No cleavage.
- Fracture
- Conchoidal, often with a waxy sheen on the broken surface
- Specific gravity
- 2.58–2.64 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm chalcedony
- Waxy rather than glassy surface — this is the key difference from ordinary quartz.
- Scratches glass, since it is quartz at a microscopic scale.
- Banding, if present, follows the curved wall of a cavity rather than running straight.
What chalcedony gets confused with
Each of these is settled by one test. Run that test first.
Surface lustre: chalcedony is waxy, ordinary quartz is glassy. Same hardness, same chemistry.
Obsidian is volcanic glass — softer (about 5), and it breaks with a glassier, more mirror-like surface.
Where it is found
Cavities in volcanic rock (agates), nodules in limestone and chalk (flint and chert), and as durable pebbles in gravel bars and on beaches worldwide.
Is it worth anything?
Common. Attractive agates with strong banding, and jaspers with distinctive patterns, sell modestly to lapidary hobbyists. The value is in the pattern, not the mineral.