oxides
Agate
SiO2
- Also known as
- banded agatefortification agatemoss agate
- Crystal system
- trigonal
- Hardness
- 6.5–7 on the Mohs scale
- Lustre
- waxy
- Colour
- Banded, in almost any combination
- Where it forms
- In gas cavities in volcanic rock, deposited in successive layers from circulating silica-rich water. The concentric banding follows the shape of the cavity wall.
- Chiefly from
- Rio Grande do Sul, BrazilIdar-Oberstein, GermanyBotswanaScotland
Why it is that colour
Bands record changes in the fluid that deposited them — different trace metals, different rates, different iterations. The pattern is a stratigraphy of the cavity filling up.
Safety
No hazard from handling.
Where it comes from
Abundant. A very large share of brightly coloured agate slices — the electric blues, pinks and greens — is dyed, using techniques perfected at Idar-Oberstein in Germany in the 1800s. Natural agate is mostly grey, white, brown and soft orange.
Historical use
Idar-Oberstein was the centre of the European agate trade for centuries, and when the local deposits ran out in the 1800s its cutters turned to Brazilian rough — which is how a German town came to define the world's agate industry and why so much Brazilian material still flows there.
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 by Theophrastus after the Achates river in Sicily, and used for amulets across the ancient Mediterranean and Near East. Ancient sources credit it broadly with protection and with strength. Modern colour-specific meanings are recent overlays.
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.
Sold as this, but not
Worth knowing
The banding in an agate is a genuine record, and reading it is a real if minor science. Each band marks a distinct episode of deposition, and the sequence preserves the order in which they happened. Some agates contain bands that formed flat under gravity — so-called water-level agates — and those bands act as a spirit level, showing which way was up when the cavity was filling.
Almost nobody buying a dyed agate slice is told that the colour is dye. It is worth knowing that the practice is nearly two centuries old and industrially standard, not a recent deception.
Identity checked against IMA-CNMNC master list, June 2025.