Large minerals are created from magma, when magma rises to the Earth's surface it cools slowly. As the magma cools, solid rocks form. Rocks are mixtures of minerals. Granite is a common rock that forms when magma cools. It contains the minerals quartz, plagioclase feldspar, and potassium feldspar. The different colored speckles in the granite are the crystals of the different minerals. The mineral crystals are large enough to see because the magma cools slowly, which gives the crystals time to grow. Small minerals are created from lava, since lava is on the Earth's surface so it cools quickly compared to magma on Earth. Hence, rocks form quickly and mineral crystals are very small. Sometimes, lava cools so hastily that crystals cannot form at all, forming a black glass called obsidian. Because obsidian is not crystalline, it is not a mineral. NOTE: Minerals (and gems) are crystals.
