Opals to back or not to back

Hi, a little canvassing of opinion here.

Where you have obviously thin bits of opal you think are still very attractive, you can make a doublet out of them. Fine.

Where you have a crystal opal that is thicker, say 2-3 mm, that could stand on it's own, are you tempted to make a doublet with a black base to bring out the colour, leave it 'as is', or is there a third option of just colouring the back of the stone dark?

Doublets must be declared as such, which is as it should be.

What is the accepted norm in the industry of colouring the back of a stone? Again, it would be declared as such, but what does it do to the value of the stone?

A solid crystal opal, vibrant colours with a dark ink / pigment on the back or the same opal without the ink / pigment; how would you value such stones? A naturally dark crystal of the same vibrancy would likely be more expensive but if a transparent crystal could be offered with a black back, would that detract from the value of offering it with no black back.

I suppose what I'm asking is will you depreciate the value of the crystal opal if you make the back black rather than add a traditional doublet back?

Or would you leave the option of the coloured back to the stone setter?!

cheers,