Colorless, milky white, hazy blue, or black with impeccable iridescence; the opal is truly unique and one of my favorite stones. The iridescence, or play of color in opal is caused from certain minerals within the stone. Black opal is considered to be rarest, whereas, white, grey and green are most common. It's Mohs hardness is 5.5 - 6.5 (Mohs hardest goes up to 10), so that means opal stones are soft enough to shave down and carve into shapes, and is also offered in most of my rings. Opals are fragile and termed as amorphous, because they lose water when exposed to air.
Fire Opal - Image courtesy of OpaleMio
Fire Opal:
Fire opal is a variant of precious opal. It has a orange and reddish hue, and is not necessarily iridescent. Fire opals are found mainly in Mexico. Red is the rarest color, so these types of opals are quite valuable.
Crushed Black Opal Stones - The Inlay
Black Opal:
Mostly found in Australia and Ethiopia. Black opal is very rare. Characterized by a dark body tone with various speckles of color (which are minerals) in it. Black opal is popular due to the black tone easily shows the speckles of color in it.
Synthetic Opals:
These opals are produced in a lab, versus having them form naturally in the earth. It shares the same chemical makeup as natural opal, however it was formed in a vacuum under pressure and extreme heat. It also has a different granular structure than natural opal. Synthetic opals are good options because of their price point and they show more brighter array of colors within the stone.