Kermesite , Calcite Locality: Pezinok , Malé Karpaty Mts, Bratislava Region , Slovakia ( Locality at mindat.org ) Size: 5.9 x 5.7 x 2.6 cm. A fine specimen of radiating sprays of flat-lying, highly lustrous, acicular, deep wine-red kermesite crystals on calcite-covered massive sulfide matrix from a classic locale - Pezinok, Slovakia. The crystals reach 2.3 cm on this piece, making for excellent material for the species and locality. Color is obvious without need for fancy lighting, and the contrasting matrix is most unusual. This is a very attractive piece with large crystals for the locale...and rare on the market today, as well. — Photo: Robert M. Lavinsky · CC BY-SA 3.0
Kermesite
Antimony oxysulfide — 'red antimony' — forming cherry-red to black needle crystals, prized since antiquity as a pigment.
Intro adapted from Wikipedia (CC BY-SA).
Kermesite is an IMA-recognized mineral species in the oxides & hydroxide class, with the chemical formula Sb₂OS₂.
It crystallizes in the triclinic crystal system (space group F-1).
Key properties: Brownish red streak; Adamantine to semimetallic luster; optically biaxial.
IMA symbol Kem, grandfathered status — known before the IMA’s founding, named in 1832.
A secondary mineral, as an alteration of stibnite, in antimony deposits.
It is found associated with Stibnite, antimony, senarmontite, valentinite, cervantite, stibiconite.
The name comes from kermes (after Persian qurmizq, crimson) for red amorphous antimony trisulfide.