What Rock is Amethyst Found In?

Amethyst is a popular and beautiful purple variety of quartz that has been used in jewelry for centuries. But what kind of rock is amethyst actually found in? The short answer is that amethyst is most commonly found in igneous, metamorphic, and sedimentary rocks. Let’s take a closer look at how amethyst forms in each type of rock.

Igneous Rocks

Igneous rocks form from magma or lava that has cooled and solidified. As magma cools, minerals start to crystallize and form crystals. If the conditions are right, quartz crystals can form, and sometimes those quartz crystals are purple amethyst. Amethyst is relatively uncommon in igneous rocks, but some examples where it can be found include granite, pegmatites, and rhyolite.

Granite is a very common intrusive igneous rock that forms deep underground. It contains quartz as one of its main mineral components. While most quartz in granite is clear or white, some formations contain purple amethyst crystal inclusions. These are usually small but can sometimes form large clusters or geodes.

Pegmatites are a very coarse-grained type of granite rock with large interlocking mineral crystals. Amethyst is more abundant in pegmatites and the crystals can grow quite large, sometimes over a meter long! These geodes lined with amethyst crystals can be prized by collectors.

Rhyolite is an extrusive igneous rock with a high silica content. It often contains quartz and amethyst is occasionally found lining cavities and fractures within rhyolite flows.

Metamorphic Rocks

Metamorphic rocks are formed when pre-existing igneous, sedimentary, or metamorphic rocks are changed by high heat and pressure. Amethyst can form when quartz crystals in the original rock undergo metamorphism. Examples of metamorphic rocks containing amethyst include gneiss, schist, and marble.

Gneiss is a banded metamorphic rock that often forms from granite. During metamorphism, new minerals crystallize and quartz inclusions can take on a purple amethyst color. Amethyst is found in some high-grade gneiss deposits.

Schist is a medium to high-grade metamorphic rock with sheet-like layers. The high pressures can cause crystallization of quartz into large amethyst crystals aligned between the schist layers.

Marble is a metamorphic rock that originates as limestone. When limestone is metamorphosed, veins or cavities of quartz and amethyst are sometimes deposited within the recrystallized marble rock.

Sedimentary Rocks

While less common than in igneous or metamorphic rocks, amethyst can occasionally form in sedimentary rocks as well. This happens when quartz crystals precipitate out of mineral-rich waters flowing through sedimentary rock. Examples include amethyst found in calcite veins in travertine or in geodes within limestone and sandstone deposits. The geodes form when groundwater dissolves limestone, leaving behind amethyst-lined cavities.

Amethyst’s purple color comes from trace amounts of iron within the quartz. It’s an enchanting and popular gemstone that can form in many different types of natural rock environments. With igneous, metamorphic, and sedimentary rocks all potentially containing amethyst, geologists have to investigate the local geology to determine where productive amethyst deposits may be found.