Emerald is the green to greenish blue variety of the mineral beryl, a mineral species that also includes aquamarine, morganite and heliodor. Beryl has the hardness of 7.5-8 on the Mohs scale. However, most emeralds are highly included so their toughness, or resistance to breakage, is considered generally poor. Purchasing a fine gem of an emerald can be a rewarding investment as long as you buy a certified gem from a trusted source, and build a sturdy, beautiful piece of jewelry around it.