NATURAL HISTORY MUSEUM, LONDON / SCIENCE PHOTO LIBRARY NATURAL HISTORY MUSEUM, LONDON / SCIENCE PHOTO LIBRARY
Rubies and sapphire. These are both varieties of the mineral corundum, which is itself a crystalline form of aluminium oxide with traces of iron, titanium and chromium. Rubies are always red, mainly due to the presence of chromium, whereas sapphires come in a number of colours.
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