Spinel stones comparable in color to rubies are quite valuable. Topaz is a silicate of aluminum containing fluorine and hydroxyl and it also has a hardness of 8. Blue topaz s are normally produced by irradiation. There are many varieties. Emerald is a well known variety that comes in the colors of emerald green, light green, yellow green, and dark green.
Emeralds can be quite expensive.
Only the finest qualities are transparent. Inclusions, healed cracks, and foreign crystals are not classified as faults, but are evidence as to the genuiness of the stone. Aquamarines are light blue to dark blue or blue-green. A deep blue is the most desirable color. Golden beryl is a lemon yellow to a golden yellow variety.