Black tourmaline, formally known as Schorl, is a widespread and abundant mineral. This opaque, deep black variety is the iron-rich end member of the tourmaline group, a complex family of borosilicate minerals. The characteristic dark color of Schorl is directly attributed to the high concentration of iron within its crystalline structure. It is found on every continent and represents the most common species within the entire tourmaline family.
The Geological Environments Where Black Tourmaline Forms
The formation of Schorl requires a high-temperature, silica-rich geological environment, often involving the presence of boron, which is necessary for all tourmalines. The most common source is granite pegmatites, which are extremely coarse-grained igneous rocks that crystallize from the final, water-rich melts of magma. Within these pegmatite bodies, the unique conditions allow large, elongated prismatic crystals of iron-rich Schorl to grow, often embedded alongside quartz and feldspar.
A second significant setting for black tourmaline is within metamorphic rocks, such as schist and gneiss. In these environments, pre-existing rocks are altered under intense heat and pressure, typically during mountain-building events. The iron and other elements in the original rock are mobilized and recrystallize into Schorl, forming small, disseminated crystals or radial clusters within the foliated rock layers.
Schorl can also form through hydrothermal processes, where hot, mineral-rich water circulates through fractures and fissures in the Earth’s crust. As these fluids cool, the dissolved iron and borate precipitate out, depositing black tourmaline crystals in veins. The presence of iron is the single most important chemical requirement, driving the formation of the Schorl species over other colored tourmaline varieties.
Major Global Production Sites
Brazil is recognized as the most significant global source of black tourmaline, primarily from the vast pegmatite fields in the state of Minas Gerais. The Cruzeiro Mine in this region is particularly famous for producing high-quality specimens, which are often found in association with other complex minerals.
Numerous African nations also contribute substantial quantities of Schorl to the international supply. Countries like Namibia, Mozambique, and Madagascar are well-known for their large deposits, often extracted from pegmatite veins and alluvial sources. These African sites are important for both commercial-grade material and fine mineral specimens sought by collectors.
In Asia, Pakistan’s Gilgit-Baltistan region and neighboring Afghanistan are known for producing fine Schorl crystals, often characterized by their well-formed, terminated shapes. Russia, particularly in the Ural Mountains and Eastern Siberia, also has significant Schorl deposits, contributing to the global availability of the mineral.
Notable Occurrences in North America
In the United States, two states stand out for their historical and current tourmaline production, including Schorl: Maine and California. Maine, which officially claims tourmaline as its state mineral, has numerous occurrences in Oxford, Androscoggin, and Cumberland counties. The Mount Mica and Dunton mines in Maine are famous for their pegmatite deposits, which yield sharp black crystals alongside colored gem varieties.
California’s primary deposits are concentrated in San Diego County, where mines like the Himalaya Mine and the Stewart Mine are located. While these sites are more famous for producing pink and green gem tourmaline, they also contain abundant Schorl within their complex pegmatite structures. Black tourmaline occurrences are also found in other states, including North Carolina and South Dakota’s Black Hills region.
In Canada, black tourmaline is commonly found in the pegmatite fields of provinces such as Ontario and Quebec. These Canadian occurrences are often sources for collectors and provide a steady supply for the local mineral collecting and lapidary communities.