cockle

Cockle is the miner's name for schorl, the iron-rich black variety of tourmaline that occurs in pegmatites and metamorphic rocks. It appears as prismatic crystals, often striated lengthwise, and ranges from opaque black to dark brown depending on iron content and weathering. The term has regional use in tin and granite mining districts, particularly in Cornwall and other hard-rock mining regions.

In the field, cockle is recognized by its hardness (7 to 7.5 on the Mohs scale), its three-sided or hexagonal cross-section, and its brittleness compared to quartz. It does not cleave cleanly; instead it fractures irregularly. Under transmitted light, schorl may show a very dark brown or opaque quality, distinguishing it from the more transparent tourmaline varieties prized by gemologists.

Occurrence and practical significance

Cockle typically forms in late-stage pegmatite crystallization and is common in granitic rocks altered by pneumatolytic processes. Miners and quarry workers encounter it as an accessory mineral in granite and pegmatite working faces. It has no commercial value as an ore or gemstone, but its presence can indicate proximity to productive pegmatite zones or guide understanding of host rock alteration.

The etymology of "cockle" in this context remains uncertain; it may derive from an older dialect term or reflect the mineral's cockle-shell-like appearance when fractured. The term is now primarily historical and regional, rarely appearing in modern technical literature outside mining heritage documentation. Geologists typically use the formal name schorl instead.