An intrusive rock is an igneous rock formed by the cooling and crystallisation of magma at depth within the Earth's crust, as opposed to an extrusive rock, which solidifies after eruption at the surface. Slow cooling underground allows time for crystals to grow large, so intrusive rocks are typically coarse-grained and visibly crystalline. Intrusive rocks are central to gemmology because the great majority of gem species, including the gem-quality varieties of corundum, beryl, tourmaline, topaz and the spodumenes, formed within or in association with intrusive bodies and their derivatives.

The intrusive-extrusive contrast

Granite, gabbro, syenite and peridotite are intrusive rocks. Rhyolite, basalt, trachyte and komatiite are their extrusive equivalents, sharing the same chemistry but cooling fast enough at the surface to produce fine-grained, often glassy textures. The grain size is the immediate visual signal: an intrusive rock has crystals visible to the naked eye, while an extrusive rock typically requires a microscope to resolve its texture. Pegmatites, the very coarse-grained late-stage intrusives, are an extreme expression of this rule.

Intrusive bodies and gem deposits

Magma rises through the crust and stalls at various levels, producing characteristic shapes. Plutons are large, deep, irregular bodies often kilometres across; stocks are smaller plutons; batholiths are composite bodies of multiple plutons covering hundreds of square kilometres. From these, smaller intrusions branch off as dykes (sheet-like, cutting across structure) and sills (sheet-like, parallel to structure). At the very latest stages, water-rich and incompatible-element-rich residual melts crystallise as pegmatites, and it is here that most gem tourmaline, beryl, spodumene and topaz are found.

Specific gem associations

Granitic pegmatites host the bulk of gem-quality beryl (aquamarine, morganite, heliodor), tourmaline, kunzite, hiddenite and a substantial fraction of gem topaz. Syenite-derived pegmatites in places such as the Kola Peninsula and Mont Saint-Hilaire produce rare collector species. Gabbroic and ultramafic intrusions are the source of peridot from mantle-derived dunite and of some chrome-bearing pyrope garnet. Carbonatites, intrusive carbonate-rich igneous rocks, host certain rare gem species including some apatite. Diamond is brought to the surface from deep mantle sources in the eruptive but ultramafic kimberlite and lamproite, which straddle the intrusive-extrusive line.

Reading geological reports

For a gem dealer reading an origin report or a geological description of a deposit, the language of intrusive rock is the basic vocabulary. Knowing that a tourmaline came from a granite pegmatite signals certain chemistry and inclusion expectations; knowing that a peridot is from a basaltic xenolith versus a mantle dunite signals different size and clarity profiles. Origin determination at the laboratory level draws on the trace-element and inclusion characteristics fingerprinted to specific intrusive host environments.