pegmatite

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A coarse-grained granite, sometimes rich in rare elements such as uranium, tungsten, and tantalum.

from The Century Dictionary.

• noun Coarsely crystallized granite. Also called granitel, granitelle.

from the GNU version of the Collaborative International Dictionary of English.

• noun Graphic granite. See under granite.
• noun More generally, a coarse granite occurring as vein material in other rocks.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A coarsely crystalline igneous or plutonic rock composed primarily of feldspar and quartz, normally with muscovite and/or biotite mica. Often contains other minerals, which may be of economic importance. Pegmatite is chemically identical to granite, but has a much coarser crystal structure. Common colors are gray, white, and pink. Pegmatite is quarried for decorative stone and as a source of beryllium, columbium and tantalum when these are present. Gemstones of the quartz/silicate family may also be found in pegmatites.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a form of igneous rock consisting of extremely coarse granite resulting from the crystallization of magma rich in rare elements