It is formed when molten magma deep within the Earth cools and hardens. It is usually a dark color. If it cools very quickly and is made up of lighter materials, it will have bubbles in it - like basalt. If it cools very slowly underground, it will not have bubbles but will have crystals. The slower the rock cools, the more crystals it will have.
Most igneous rocks have crystals.
An important rock for the Chumash and other Indians was obsidian - a glass formed from rapidly cooled lava. It formed so quickly that there is no time for crystals to grow. Although there is a lot of volcanic material at Stunt Ranch, there is no obsidian. Granite forms as a volcanic material deep under the earth and cools very slowly. It is composed of quartz, silica and oxygen.