May Birthstone - Emerald Roger K. Pabian Emerald is used to celebrate the 55 th Anniversary according to the World Almanac and Book of Facts . It is probably suitable that such a rare gem as emerald is used for this event as few events or happenings, aside from birthdays, reach this many years.
Fine emeralds are probably among the rarest of gems.
Crystals are hexagonal, 7.5 to 8 hard, 2.65 to 2.75 in specific gravity. Emeralds, as all other beryls, can range from being opaque to transparent. Beryl has fairly low optical properties. The refractive index ranges from 1.57 to 1.60 with many gems falling in the 1.572 - 1.580 range. Beryl has a low birefringence and are only weakly doubly refractive.