Black opal is the most coveted of all opal varities. The "black" in black opal means the gem has a very dark body tone. A black opal can be crystal or opaque. The rarest colour is red, next is green and orange with blue as the most common colour. Semi-black opal has an inherent darkness in its body color when viewed against a white background. Mid-grey stones are termed as semi-black.
There are two categories of opals - common and precious. The Precious opals are composed of three groups: white, black, and fire opal.Fire opal are: Brings out spontaneous action Initiates new ideas Stimulates enthusiasm Acts as a magnet to draw in money or customers to a business