Lesson 20: Pegmatites and Tourmaline
By the end of these three Lessons (Lessons 20-22) on pegmatites and pegmatite gems,
Describe the diagnostic properties of the three main pegmatite gems varieties:
tourmaline, topaz, and spodumene
Describe the basic crystal chemistry of tourmaline, topaz, and spodumene
Explain the mineralogical differences between the different tourmaline and
spodumene varieties (e.g., rubellite and kunzite)
Describe the external morphology of tourmaline, topaz, and spodumene crystals
Define the main variables used to value gem tourmaline, topaz, and spodumene
Describe the term "pegmatite"
Describe the geological setting and physical morphology of gem pegmatites
Describe the global and local distribution of gem pegmatite
If you want to find gemstones in the rough, the best rock type to concentrate on is
pegmatite. Pegmatites supply the world with the best tourmaline, topaz, and beryl along
with a large selection of other rare stones - some so rare that their faceted varieties are
only cherished by the few collectors who can get their hands on them. In addition to the
wonderful gems that pegmatites produce, these rocks are also important hosts for rare
metal deposits, including lithium (Li), tantalum (Ta), niobium (Nb), and tin (Sn).
Pegmatites are known for growing the largest crystals and are the environment for many
"largest crystal" records for specific minerals. In fact, the term pegmatite is used as a
descriptor for igneous rocks with large crystal sizes.
In these Lessons on pegmatite, we'll first look at the different types of minerals hosted in
pegmatites and then take a closer look at tourmaline, spodumene, and topaz. Later, we'll
describe the environments where pegmatites occur, and look at some specific examples in
the USA and Canada.