They drop four ash when broken, and have a smoke-like particle effect above them. The resulting ash can be used in a furnace resulting in 2 smelting processes per item making them as efficient as coal. Also, you can craft coal by putting it into a crafting table in a 3x3 pattern.
You can also use ash to create Gray Dye. One block of ash will create two bowls of Gray Dye.
In 1.12.2-22.214.171.1242, players and mobs no longer have a slowdown on this block.