Pelona — A Coahuila Source for Faceting-Grade Calcite
Pelona — A Coahuila Source for Faceting-Grade Calcite
A northern Mexican locality known to mineral collectors for transparent honey-yellow and pale pink calcite crystals
Pelona is a mining locality in the state of Coahuila in northern Mexico, recognised in the international mineral collector market for its production of gem-quality calcite. The deposit yields transparent to translucent calcite in honey-yellow, pale pink, and colourless habits, with crystals of a size and clarity sufficient to fashion faceted stones intended primarily for the collector cabinet rather than for jewellery.
The mineral
Calcite is calcium carbonate, CaCO3, crystallising in the trigonal system. It is one of the most common minerals at the Earth's surface, but transparent crystals of facetable size and clarity are uncommon. Calcite is exceptionally soft, with a Mohs hardness of 3, and shows perfect rhombohedral cleavage in three directions. The combination of low hardness and easy cleavage rules calcite out of mainstream jewellery use; an unprotected ring stone of calcite will not survive ordinary wear. The species is also strongly birefringent — refractive indices of 1.486 and 1.658 — which produces the dramatic doubling of pavilion facets that is calcite's most recognisable optical signature.
The deposit
Pelona calcite forms in hydrothermal vein and vug environments associated with the carbonate sediments and intrusive rocks of the Sierra Madre Oriental. Crystals are typically scalenohedral or rhombohedral in habit, with single individuals reaching sizes that allow facet rough up to several hundred carats. Honey-yellow Pelona material has been featured in Gems & Gemology as among the finest faceting-grade calcite recovered in recent decades, alongside material from Iceland (the type locality for optical-grade Iceland spar) and from older deposits in Britain and Mexico's neighbouring states.
Cutting and use
A faceter approaches calcite with care and patience. The cleavage demands gentle dopping and slow grinding; the softness rules out aggressive lap speeds and is the reason most calcite is fashioned with cerium oxide on a tin or composition lap rather than the harsher abrasives used for quartz and harder stones. Designs that take advantage of calcite's birefringence — open table cuts, simple step pavilions — show the doubling effect to good advantage. Cut calcite is sold to mineral and gemstone collectors who maintain reference suites of unusual species, to museums building teaching collections, and occasionally to designers willing to set a soft stone in a protected pendant or earring.
In the trade
Pelona calcite is rarely encountered in mainstream coloured-stone supply chains. The market is collector-to-collector, with major dealers at the Tucson and Munich shows and through specialist mineral auctions. Buyers should ask after the locality by name, since calcite of comparable colour comes from numerous deposits and the Coahuila origin is part of what gives Pelona material its small premium.