Heat rocks

From UroWiki

One widely-held rule in uromastyx circles is to never use heat rocks or heat caves, or similar items (also including under-tank heaters. Although uromastyx will normally bask on hot rocks, this is not because they enjoy the warmth below them as a human might, as they have no thermal sensors on their undersides. In fact, the reality of the rock being hot is simply a coincidence due to the fact that it is the place where emissions from the sun (or heat lamp) are strongest - from above.

Commercial hot rocks

Not only do uromastyx not need such intense temperatures from below, but some commercial heat rocks have been known to malfunction and heat up too much, and since the uromastyx lacks thermal sensors on its underside, it cannot tell when it is too hot: the heat rock will literally cook the lizard from below. Obviously this is a horrible fate, which can leave your uromastyx either dead, or with severe burns which will cause pain as well as extensive vet bills for treatment.