Mineral crystals grow under a variety of conditions. Slow cooling of magma (melted rock) deep within the Earth generally leads to a rock such as granite with visible crystals of feldspar, quartz, mica, amphibole and other minerals. Water in the magma can even facilitate pegmatite formation. Pegmatite is an igneous rock composed of mineral crystals generally with a diameter of several centimeters which, on rare occasions, can be meters.