Opals are hydrated silicon dioxide that means they contain large amount of water in their composition. Low quality opals lose water gradually and become brittle and prone to cracking after some time. Applying water intermittently prevents loss of water and protects the stone. High quality opals do not need water and are inherently resistant to drying.
Opals are generally categorized as precious and non-precious, though even non-precious opals are valuable but precious opals are much rarer and commercially much more expensive.
Precious opals exhibit play of colors and color flashes. Different colors are seen as the stone is viewed from different angles.
The color play phenomenon of precious opal is believed to be caused by microscopic sheets that are located at different angles inside the opal and that explains the occurrence of color play and flashes.
There are some varieties of precious opal like crystal opal, boulder opal, white opal and black opal.
Crystal opal lets light pass through and is translucent. White opal is essentially crystal opal but more turbid (milky).
Boulder opal is crystal opal mixed with another stone (usually iron oxide).
Black opal is like crystal opal but the matrix color is darker that lets color play seen much more prominently.
Normal opals are relatively abundant but precious opals have only been mined only in few countries like Ethiopia and Australia.
Opals from Ethiopia are nearly always in form of crystal opal.
Australia recently is the supplier of crystal, boulder and black opal.