Pegmatite is found in all parts of the world, and they are most abundant in very old rocks. Some are found in large intrusive igneous rocks, while others are distributed in the rocks surrounding the intrusive igneous rocks.
Pegmatite is almost always a wholly crystalline igneous rock that has very coarse grain. The average size grain for pegmatite is 3 to 4 inches. According to Encyclopedia Britannica, most pegmatites are tabular, shaped like a cigar or have an irregular shape. Granitic and syenitic pegmatite deposits are the main source of commercial feldspar, sheet mica and beryllium, tantalum niobium and lithium minerals. Pegmatites also produce a significant amount of gem minerals such as mica, molybdenite and cassiterite.