Aaron Leonard, age l0, Of Tucson, Ariz., for his question:

How is obsidian formed?

The mineral called obsidian is glossy black glass. It is an igneous or fire formed rock made by volcanic fury in the earth's crust. In the fiery fury, quartz minerals were boiled and melted. Quartz is the hardest of the cinmanon minerals. It makes up most of the gritty grains of sand, and quartz sand is one of the ingredients used in man~made glass.
A few million years ago volcanoes raged in our western mountains. some of them poured forth rivers of molten quartz minerals. And some of this seething lava cooled into glassy obsidian. In many places, lumps and great chunks of obsidian are found on the ground and in a few places whole cliffs are made of this natural glass.