Emerald is a variety of beryl, a beryllium aluminum cyclosilicate (Be3Al2(SiO3)6) of which the crystal system is hexagonal. The varieties of beryl occur in many colors, but emerald always occurs in hues of green. The green coloring 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, columnar emerald here crystals here seen, inside a quartzitic host-rock, measure up to 60 mm in length. Sample is from long-familiar mines in Muzo, Colombia