An LCT pegmatite is a rare-element pegmatite enriched in lithium (Li), caesium (Cs) and tantalum (Ta), and is one of the two principal classes of rare-element granitic pegmatite recognised in the modern petrological literature, the other being NYF pegmatites enriched in niobium, yttrium and fluorine. From a gemmological standpoint LCT pegmatites are the geological setting that produces a great deal of the world's finest tourmaline (especially elbaite), much of the gem-quality lithium beryls (morganite, bicolour beryl), kunzite, hiddenite, tantalite-columbite group minerals (rare collector species) and a long tail of associated rarer phosphates and silicates.

LCT pegmatites form during the latest stages of crystallisation of granitic magmas, in particular peraluminous, S-type granites derived from the partial melting of metasedimentary rocks. As the parent granite crystallises, incompatible elements (lithium, caesium, tantalum, beryllium, fluorine, boron, phosphorus and others) are progressively concentrated in the residual melt; when this melt reaches a saturation point, it is expelled into fractures in the surrounding country rock, where slow cooling under volatile-rich conditions produces the giant crystals characteristic of pegmatites. The internal zonation of an LCT pegmatite typically progresses from a quartz-feldspar wall zone to a more lithium-rich intermediate zone, to a final pocket or core zone enriched in the most incompatible elements; gem material is concentrated in the pocket zone, especially where late-stage hydrothermal solutions have re-mobilised and re-crystallised earlier minerals.

The world-class LCT pegmatite districts that supply the gem trade include the Tanco mine in Manitoba, Canada (a benchmark caesium-tantalum producer that has also yielded beryl and lithium minerals); the Minas Gerais belt in Brazil (Itinga, Aracuai, Galileia, the Sapucaia pegmatite, the Cruzeiro mine), source of much fine elbaite tourmaline and aquamarine; the Pala district of California (Stewart Lithia and Tourmaline Queen mines), historically important for kunzite and pink tourmaline; Afghan and Pakistani belts (Nuristan, Paprok, Gilgit-Baltistan), source of fine kunzite, morganite and tourmaline; and the Madagascar belt, source of much liddicoatite and elbaite.

For the gem trade the practical relevance of the LCT designation is that it explains, in a single label, why so many different gem species cluster at the same localities. A tourmaline mine that produces bicolour elbaite will, almost without exception, also produce occasional morganite, kunzite, lepidolite-mica matrix and rare phosphate species, because all of these reflect the same residual-melt enrichment in lithium and other incompatible elements. Understanding the LCT geological setting is also useful for origin determination work in laboratories, as trace-element fingerprints from LCT-hosted gems differ in characteristic ways from those of metamorphic or hydrothermal sources.