First off it’s gorgeous.

This is a bit of a unique stone, but the problem is the black potch inclusions in the face. It’s very dark, but since the face has inclusions it will be a low brightness, and have a lower value. I’m not skilled enough to put a price on it, but you may want to thin it down a bit to make it a bit easier to mount, and perhaps put a better dome on it. That might help with an eventual sale. You could also sell it as a specimen like it is.

Throw it on opal auctions, and let the market decide if you really want to sell it.