Precious Opals are often categorized into several type which I will list and briefly describe below

  1. Black Opals are Precious Opals with a black or gray background color. These opals are usually translucent to opaque.
  2. White Opals are Precious Opals with a white or light background and are usually translucent to opaque.
  3. Crystal Opals have more transparency, usually semitransparent.
  4. Water Opals are even more transparent than Crystal Opals, usually transparent or close.
  5. Boulder Opals are Precious Opals which are left connected to the host stone or matrix which is usually ironstone.
  6. Yowah Nut Opals and Koroit Opals are fairly rare and are specific types of Boulder Opals.
  7. Fossilized Opal are voids left by plants or animals which were then filled with silica gel long ago, eventually hardened and fossilized
  8. Composite or assembled opals are doublet and triplets which have other materials added to them to make them prettier, more durable or both.

There may be other types that are not on this list. I will white a bit more about each of these in more detail.