oxides
Ruby
Al2O3
- Also known as
- pigeon blood ruby
- Crystal system
- trigonal
- Hardness
- 9 on the Mohs scale
- Lustre
- adamantine
- Colour
- Red, from pinkish to deep blood red
- Where it forms
- In marbles formed where limestone was metamorphosed with aluminium-rich impurities, and in some basalts. The Burmese marble-hosted deposits produce the classic material.
- Chiefly from
- Mogok, MyanmarMontepuez, MozambiqueThailandMadagascar
Why it is that colour
Chromium substituting for aluminium. The same element that makes emerald green makes corundum red, because what a transition metal does to light depends on the structure it sits in as much as on the metal.
Safety
No hazard from handling. Most commercial ruby is heat-treated, and some is lead-glass filled, which is fragile and dissolves in household acids such as lemon juice.
Where it comes from
This is the hardest supply chain in the collection to defend. Mogok in Myanmar has produced the finest ruby for centuries under conditions that have included forced labour, and Burmese stones have been sanctioned by the United States at various points. Mozambique’s Montepuez deposit, opened in 2009, has been the subject of serious allegations about treatment of artisanal miners, and its operator paid a settlement to Mozambican claimants in 2019.
Historical use
Among the most valuable gems by weight, exceeding diamond at the top of the market. The stones in the British Imperial State Crown long called the Black Prince’s Ruby and the Timur Ruby are both spinel, not ruby, and the confusion was not resolved until the eighteenth century.
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
One of the most consistently documented stones in the world. Sanskrit sources call it ratnaraj, king of gems. It appears in the Hebrew Bible, in Burmese warrior tradition — where stones were said to be inserted under the skin for invulnerability — and throughout medieval European lapidaries as a stone of blood, courage and vitality. The modern associations largely continue the ancient ones.
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 spinel confusion is one of the great naming errors in the history of gems, and it lasted centuries. Red spinel and ruby occur together, look identical to the eye, and were both called ruby. The Black Prince's Ruby, set in the British Imperial State Crown and carried through Agincourt by Henry V, is a spinel. So is the Timur Ruby. Nobody could tell until the chemistry was worked out in the 1780s.
There is a lesson in it that applies well beyond gemstones. For six hundred years the most carefully guarded and most closely examined stones in Europe were misidentified, by experts, unanimously, and the error was only visible from outside the system that produced it.
Identity checked against IMA-CNMNC master list, June 2025.