What gem is color green?

Emerald/Peridot are the true green natural gemstones.

The Emerald cut has 58 facets (25 crown, 8 girdle and 25 pavilion). The most common and usually most valuable green stone is the Columbian Emerald which is usually ground/polished to the emerald style cut.

The crystal structure of a true Columbian Emerald also gives it allot of extra brilliance. In a picture this looks like defects but if you look closely you see all the facets are still flat in a flawless Columbian. It is when you have eye visible black carbon, facet chips or crack that you have defects. Everyone knowledgeable wants the crystal for the brilliance.

In ancient Egypt, Peridot was mined on an island called Zeberget. Mining was done at night because legend said that Peridot could not be easily seen during the day, but its pale iridescent green was easily illuminated by lamplight at night. It was for this characteristic that the Romans called Peridot the "evening emerald." Peridot later was also often used to decorate medieval churches, and was probably carried back to Europe by the Crusaders. Large Peridots, more than 200 carats in size, adorn the shrine of the Three Magi at the Cologne Cathedral.

Some historians even suspect that at least some of the "emeralds" worn by Cleopatra were actually Peridot.