This piece of Koroit boulder Opal is exceptional under the torch light from my phone. In daylight, it has lovely visible greens and blues on the face. I am writing this in the dark. Sometimes I work best by cover of nightfall...
 
This piece of opal came from the Koroit region of Australia by way of TwoSkies, who then sold it to me. It has passed from rockhound to rockhound and could be yours next. 
 
This opal is still in the iron-rich boulder material. It has blues and greens on the face but whne you flip it over to the rough side, you can see greens and blues threaded through the bumps. It's actually splendid - and that is not a word I use. 
 
This Koroit boulder opal mineral specimen isn't technically a crystal because no opal is, it's more of a fossil that became pretty over eons.  However, opal remains everyone's favourite shiny. 
 
This opal is a rough dropped diamond shape. One side has been part polished but the other side is the prettiest. I'm tempted to keep this one now that I have noticed it is dual sided. The only thing that is stopping me is that it takes a minute to write descrptions and I don't like wasting time.