NATURAL HISTORY MUSEUM, LONDON / SCIENCE PHOTO LIBRARY NATURAL HISTORY MUSEUM, LONDON / SCIENCE PHOTO LIBRARY
Citrine and sapphire gemstones. Fine and poor examples of gem cutting and polishing. At left is a superbly cut citrine (a yellow variety of quartz), while at right is a poorly cut sapphire (blue), a variety of the mineral corundum (aluminium oxide).
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