If you guessed this one, you're right. This is a piece of pumice, a type of rock that forms when lava quickly cools on the surface of the earth.
When lava cools quickly, it doesn't have a chance to form the large crystals you find in many other rocks, such as quartz. In the case of pumice, the lava rapidly cools into a lightweight material full of holes, or pores. Those holes, full of air, are what keep pumice afloat.
Because they originate from lava, both granite and pumice are truly "born of fire." Both are known as igneous rocks.