Pleonaste — The Iron-Rich Dark Variety of Spinel
Pleonaste — The Iron-Rich Dark Variety of Spinel
Ferroan spinel from Sri Lanka and Myanmar in the collector's market
Pleonaste is the dark brown to black variety of spinel rich in iron and magnesium, with the approximate formula (Mg,Fe)Al2O4. It represents the iron-rich end of the spinel-hercynite solid-solution series and typically exhibits lower transparency and lustre than the gem-quality magnesium-aluminium spinel that dominates the modern coloured-stone trade. The name comes from the Greek pleonastos, meaning superfluous, in reference to the multiplicity of crystal faces shown by some specimens. Pleonaste is largely a collector's stone today and is rarely encountered in commercial fine-jewellery channels.
Composition and structure
The spinel mineral group is defined by the AB2O4 structure, with cations distributed across the A and B sites. In gem spinel the A site is dominated by magnesium; in pleonaste, iron substitutes for magnesium in significant quantities, shifting the chemistry toward the hercynite end-member (FeAl2O4). The iron content darkens the colour and reduces the transparency relative to magnesium-rich spinel.
Crystallographically, pleonaste is cubic, with a hardness of 7.5 to 8 on the Mohs scale, a specific gravity around 3.6 to 4.0, and a refractive index of approximately 1.78. The optical and physical properties overlap those of standard spinel except for the deeper colour and reduced transparency.
Historical sources
Classic pleonaste has been mined from the Ratnapura gem gravels of Sri Lanka, where it occurs alongside other spinel varieties, corundum, and chrysoberyl. The synonym ceylonite derives from the historical name for Sri Lanka and is occasionally encountered in older mineralogical literature. Myanmar (Burma) has also produced pleonaste from the Mogok area, where the spinel mineralogy is among the most diverse in the world. Other documented occurrences include localities in India, Madagascar, and Vietnam.
Cutting and trade position
Because of its dark tone and reduced transparency, pleonaste is rarely faceted for fine-jewellery use. Material with sufficient transparency may be cut en cabochon to display the depth of colour without revealing internal cloudiness; collectors' stones may be faceted to demonstrate the chemistry. Pleonaste is more typically encountered as crystal specimens in mineralogical collections than as cut stones in trade channels.
The species sits outside the modern fine-jewellery spinel market, which is dominated by the saturated red, pink, and blue varieties from Mogok and Mahenge that have driven the spinel renaissance of the past two decades. Buyers seeking pleonaste are usually mineral collectors or specialists in the historical literature rather than fine-jewellery clients.