DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Emerald is a variety of beryl, a beryllium aluminium cyclosilicate (Be3Al2(SiO3)6) of which the crystal system is hexagonal. The varieties of beryl occur in many colours, but emerald always occurs in hues of green. The green colouring has to do with trace amounts of chromium in the composition. When pure and flawless, emerald is the most precious green gemstone known, the deeper the green the more desirable. Emeralds are rarely flawless, but treatments may cover up some flaws, a fact implicating that stones with small flaws are sometimes valued above normal for their authenticity. The rough emerald here seen, inside quartz host-rock, measures 25 mm in length and is from long familiar mines in the Boyaca region of Colombia.
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