IF I WERE TO SELL THIS PIECE OF ROUGH OPAL ROCK, WHAT WOULD I GET OR WHAT CAN I CHARGE FOR IT? JUST CURIOUS, AND CAN SOMEONE CONFIRM IF THEY THINK ITS BLACK OPAL?
Yep it's boulder opal. It's a naturally occuring thin strip of opal on an ironstone backing/rock. Yours contains the most common blue/green colours; its large but if you wanted to cut and polish it you'd have to split it up into a few pieces (the best ones have no ironstone in face up view).
It is gorgeous (and definetly something I'd love to own)!
For selling you should look at the site posted above, to see what similar pieces go for. I've gotta reiterate again; it's so gorgeous!!!
Black Opal has a very dark grey (almost black) background. Yours is boulder Opal and a small piece. There's not enough "gem Opal" recoverable as it's just a thin streak of Opal in the ironstone. A mineral collector would pay maybe a few hundred dollars because it's very pretty.