We just learned about the metalloid semiconductor called Silicon.
One type of rock found on the earth is Obsidian.
Obsidian is a dark black glassy igneous rock that is made from lava.
By itself it is not a chemical element on the Periodic Table, it is usually made up of other elements combined, like Silicon, Magnesiu, Iron and Oxygen.
(from: wikipedia - obsidian)
Kid Facts - Blast from the past: Electrodes