If you have turquoise that is hard enough to take a polish and is not stabilized you will ruin the value of the stone by coating it. If it's stabilized turquoise then it's already infused with a plastic polymer or chemically silicated and it shouldn't require a coating.
I would avoid epoxy due to yellowing. If your set on coating the stone look into Paleo Bond Penetrant. I recently switched to their products for my inlay work.
Rocky Mountain Wonders