schorl

Schorl is the black to very dark brown variety of tourmaline, the most common color form of this borosilicate mineral. It is an iron-rich member of the tourmaline group, and its color results from iron-bearing substitutions in the crystal structure. The ideal chemical formula is NaFe3Al6(BO3)3Si6O18(OH)4, though the composition varies with substitutions of other elements such as Mg, Mn, Li, Ca, and Al.

Schorl crystallizes in the hexagonal system and commonly forms elongated prismatic crystals with a characteristic pseudo-hexagonal outline. It has a vitreous luster, a hardness of 7 to 7.5 on the Mohs scale, and a specific gravity around 3.0 to 3.2. The mineral typically shows no true cleavage and fractures conchoidally or unevenly. Due to its iron content, schorl is opaque to translucent and exhibits strong pleochroism, often appearing uniformly dark when viewed in different crystallographic directions.

Occurrence and formation: schorl is widespread and found in a variety of geological environments, including granitic and pegmatitic rocks, metamorphic rocks such as schists, and some basaltic rocks. It commonly occurs as accessory crystals in granite pegmatites and is often associated with other tourmaline varieties, quartz, feldspar, and mica. Notable localities include several producers in Brazil, Africa, Asia, and North America.

Name and classification: the term schorl is the traditional name for the black tourmaline variety, with its exact etymology tied to historical European mining districts. As a common and widely distributed mineral, schorl is widely collected as a specimen and is occasionally used in lapidary, predominantly in sculptural carvings or as beads where opaque black color is desired.