peg·ma·tite 
(pĕgmə-tīt′)
(pĕgmə-tīt′)n.
A coarse-grained granite, sometimes rich in rare elements such as uranium, tungsten, and tantalum.
[Greek pēgma, pēgmat-, something fastened together (from pēgnunai, to fasten; see pag- in the Appendix of Indo-European roots) + -ITE1.]
peg′ma·titic (-tĭtĭk) adj.