oxides
Cassiterite
SnO2
- Also known as
- tinstone
- Crystal system
- tetragonal
- Hardness
- 6–7 on the Mohs scale
- Lustre
- adamantine
- Colour
- Brown to black, rarely near colourless
- Where it forms
- In high-temperature veins associated with granite, and concentrated in placers because it is dense and hard.
- Chiefly from
- Cornwall, EnglandBoliviaBangka and Belitung, IndonesiaRwanda and DRC
Why it is that colour
Iron and other impurities; pure tin oxide is colourless.
Safety
Tin oxide is essentially inert and non-toxic. Safe to handle.
Where it comes from
One of the 3TG conflict minerals — tin, tantalum, tungsten and gold — named in the Dodd-Frank Act because their trade financed armed groups in the eastern Democratic Republic of the Congo. Indonesian production from Bangka involves large-scale unregulated dredging with documented fatalities and reef damage.
Historical use
The ore that supplied Bronze Age tin. Cornwall and Devon were mined for it from antiquity into the twentieth century, and the last Cornish tin mine, South Crofty, closed in 1998 — though rising tin prices have prompted attempts to reopen it. The Cornish mining landscape is a UNESCO World Heritage site.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
Worth knowing
Cornwall's tin is one of the longest-running industrial stories in Europe. It was being traded to the Mediterranean in the first millennium BC — Greek sources refer to the Cassiterides, the tin islands, and the name of the mineral comes from the same root. It sustained an industry for something like three thousand years, powered the Cornish beam engine, exported Cornish engineers and mining methods worldwide, and ended in 1998.
The 3TG designation is worth understanding as a policy experiment. Section 1502 of Dodd-Frank required US-listed companies to report whether their tin, tantalum, tungsten or gold came from conflict-affected areas of central Africa. It has been criticised from both directions — for imposing costs without ending the conflict, and for causing a de facto embargo that harmed legitimate artisanal miners — and the evidence on its effects remains genuinely contested.
Identity checked against IMA-CNMNC master list, June 2025.