xenocryst

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

• noun A crystal foreign to the igneous rock in which it occurs.

from The Century Dictionary.

• noun In petrography, a term proposed by Sollas for a crystal in an igneous rock which has been included by the igneous magma before the solidification of the rock.

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

• noun geology a crystal that does not occur in a specific igneous rock