sandy - if you're probing onto the onyx, the white paint will definitely throw it off & do weird things. For a while I had black paper between the onyx & the glass. That was OK, but I've since painted the bottom of the glass with high temp spray point (for BBQ grills) and it made my results much more repeatable and consistent.
I highly recommend it, I did two coats right onto the glass, then baked it in the oven to cure it.