oxides
Chrysoberyl
BeAl2O4
- Also known as
- alexandritecat's eyecymophane
- Crystal system
- orthorhombic
- Hardness
- 8.5 on the Mohs scale
- Lustre
- vitreous
- Colour
- Yellow-green, and green-to-red as alexandrite
- Where it forms
- In granite pegmatites and in metamorphosed ultramafic rocks. Alexandrite requires beryllium and chromium together, which is as unlikely a combination as the one that makes emerald.
- Chiefly from
- Ural Mountains, RussiaMinas Gerais, BrazilSri LankaTanzania
Why it is that colour
Iron gives the ordinary yellow-green. Chromium produces alexandrite, whose colour change is a real optical phenomenon: the mineral transmits in both the green and the red parts of the spectrum, and which dominates depends on the light source.
Safety
Safe to handle. As a beryllium mineral, cutting it produces beryllium-bearing dust and requires extraction.
Where it comes from
Russian Ural alexandrite is long exhausted and antique stones command extraordinary prices. Brazilian material from the 1987 Hematita find and Sri Lankan and Tanzanian material supply the modern trade. Synthetic alexandrite is common and much cheaper.
Historical use
Third hardest of the commonly encountered gem minerals, after diamond and corundum. Cat’s eye chrysoberyl is the stone that has the strongest claim to the plain name cat’s eye; any other stone showing the effect must be qualified.
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
Alexandrite was found in the Urals in the 1830s and named after the future Tsar Alexander II, reportedly because it changed between the Russian imperial military colours of green and red. That is a pleasing story with an 1830s date on it, and no older attribution exists. Cat’s eye chrysoberyl has a longer history in South Asia as a protective stone.
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
Alexandrite is one of the few stones that does something visibly strange, and the mechanism is well understood. Chromium in the chrysoberyl structure creates two transmission windows, one in the green and one in the red, with strong absorption in the yellow between them. Daylight is rich in blue and green, so the stone reads green; incandescent light is rich in red, so it reads red. The eye, lacking a reference, sees a colour change rather than a change in illumination.
This is worth contrasting with the properties usually claimed for stones. Alexandrite genuinely does something, that something is explicable, and the explanation makes it more interesting rather than less. It also carries almost no metaphysical reputation, because it was found in 1834 and the correspondence systems were already full.
Identity checked against IMA-CNMNC master list, June 2025.