Mastic is a sweet, slightly licorice-flavored resin of the Lentisk Bush
that grows on the island of Chios and in other areas of the Mediterranean. It is found in small, clear or amber-colored pebble-like crystals in Middle Eastern markets. A Greek liqueur, Masticha, made only on the island of Chios, is flavored with it. There is no substitute flavoring for Mastic.
This info came from the very wonderful "The Von Welanetz Guide To Ethnic Ingredients"