Personally I would crop out the plasticine as it is a little distracting being bright green. Nice crystal though, what does it look like with a bit of light shining through from the back?

beautiful

There is all so an other way instead of crop (cropping will loose the lowest part of the crystal & it would look "ugly".
Set your light metering to "spot" (so your camera gets light only from the crystal).
That way you increase the shutter speed & the background gets all black.
Lean you crystal with the following way...
Use a toothpick & 2 small pieces of your adhesive tack & place them "behind" the crystal.

Ummm, Ray? That IS with light shining from the back. Thanks for the tip, Karaflazz. Very useful. I'll give it a try.