According to the National Park Service:
[…] highly gas-charged molten rock cools and solidifies during flight and rains down to form cinder cones. If you look closely at cinders you will see that they are laced with gas holes and resemble a sponge.
The shiny parts are magma that has fused into glass, perhaps with some traces of metals.
I found this lava chunk as I was hiking up Inferno Cone. It was very light weight because of all the bubble cavities.
August 6, 2010. Craters of the Moon National Monument, Idaho.