This is out of "Prentice Hall Science Explorer" MINERAL COMPOSITION AND COLOR "Rocks are made of mixtures of minerals and other materials. Some rocks contain only a single mineral. Others can contain...
Slate, gneiss >_< go crazy
Obsidian is a high Silica rock (SiO2) related to Acidic extrusive igneous rocks like Andesite and Rhyolite.
Volcanic glasses have no crystalline structure, and technically no minerals.
Limestone or Sandstone are two examples of sedimentary rocks.