Heat rock

I believe heat rocks have directions specifically stating that what you propose is a fire hazard and not safe. Heat rocks in general are a terrible idea, especially for amphibians.

I can't believe they still make those horrible things.

The heating pads are fairly cheap at petsmart/petco and a much safer heating source. Given that they are on the outside of the tank, you do not have the worry of possible overheating/fire danger. I would highly suggest getting a heating pad (that sticks to the side) that is suitable to your tank size.

Remember, heat rises naturally, and for little White's, they need to be able to adjust their body temp as necessary. I find that when my Whites' are in need of a little more heat, up they go above the "warmed" side of the tank, and if they are in need of some cooling down, I find them sleeping in limbs closer to the cooler side.

I hope you get a heating pad! Better safe than sorry!!!