Opal is a bit different due to inclusions, but nice effect if polished smooth.

I make whimsical critters out of Premo and also teach a variety of folks how to sculpt them, both inexperienced and experienced. I buy Premo for both because it comes in a variety of colors and for the most part performs well, it is easy to condition, and not too sticky. With that said...the Premo Opal clay includes small flakes of an iridescent film, that will catch on your blade if you try to cut thru it, so you may need to take extra care in smoothing the surface. However, I have found that the surface will sand smooth and produce a very nice effect. Blueberry Beads has become my supplier of choice since they have a great discount for bulk purchases, have the clay I need in stock and ship very quickly!