Hackmanite

Posted on February 28, 2017 by John J. Bradshaw, GG (GIA)

Introduction Hackmanite, Na8Al6Si6O24(Cl2,S), is the rare variety of the mineral species sodalite. Sodalite is a translucent to opaque, dark blue to violet-blue material that is used for jewelry purposes, typically fashioned as cabochons or beads, while…

Gem Notes

Clinohumite

Posted on January 2, 2017 by John J. Bradshaw, GG (GIA)

Introduction Clinohumite, (Mg,Fe)9(SiO4)4(F,OH)2, is a mineral species in the humite group which also includes humite, chondrodite, and norbergite. These species are easily confused with each other as they have similar properties that often overlap. The relative…

Gem Notes

Kornerupine

Posted on November 1, 2016 by John J. Bradshaw, GG (GIA)

Introduction Kornerupine is a mineral species with the formula Mg3Al6(Si,Al,B)5O21(OH). It crystallizes in the orthorhombic system. The minerals kornerupine and prismatine were once considered to be the same mineral, but microprobe analysis proved that the two…

Gem Notes

Andalusite

Posted on July 1, 2016 by John J. Bradshaw, GG (GIA)

Introduction Andalusite is a mineral species crystallizing in the orthorhombic system with the formula Al2SiO5. Two other gem species, namely kyanite and sillimanite, have the same chemical composition as andalusite but with different physical structures. All three form under different conditions of temperature and…

Gem Notes

Diopside

Posted on April 29, 2016 by Jon C. Phillips, GG (GIA), CG (AGS)

Introduction Diopside is a mineral species crystallizing in the monoclinic crystal system with the formula CaMgSi2O6. It is the end member of a solid solution series to hedenbergite where the magnesium is replaced by iron, CaFeSi2O6. There are a few intermediate…

Gem Notes

Prehnite

Posted on February 27, 2016 by John J. Bradshaw, GG (GIA)

Introduction Prehnite is a mineral, well known to collectors, with the formula Ca2Al2Si3O10(OH)2. Crystals are rare, typically forming with stalactitic or botryoidal habit (Fig. 1). It is not commonly seen in jewelry stores but is occasionally…

Gem Notes

Apatite

Posted on December 29, 2015 by John J. Bradshaw, GG (GIA)

Introduction Apatite is a group of closely related species, namely fluorapatite Ca5(PO4)3F, chlorapatite Ca5(PO4)3Cl, and hydroxylapatite Ca5(PO4)3(OH). Fluorapatite is by far the most common species of the apatite group. Almost all gemstones are fluorapatite although rarely…

Gem Notes

Fluorite

Posted on November 2, 2015 by John J. Bradshaw, GG (GIA)

Introduction & History Fluorite is calcium fluoride (CaF2). It is well known in the mineralogical world and is often seen as well-formed crystals or crystal groups. Fluorite’s low durability has lessened its importance in the world…

Gem Notes

Benitoite

Posted on August 31, 2015 by John J. Bradshaw, GG (GIA)

Introduction and History: Benitoite is barium titanium silicate, BaTi(Si3O9). Mindat.org lists several localities for benitoite including the Czech Republic, Japan, Arkansas and Montana, but it is well known that the only source of gem quality…

Gem Notes

Tugtupite

Posted on June 30, 2015 by John Bradshaw, GG (GIA)

Introduction: Tugtupite is an interesting and rarely seen gem, closely related to sodalite with the formula, Na4(AlBeSi4O12)Cl. It was discovered in 1957 by Professor H. Sørensen in the coastal cliffs of Tugtup Agtakôrfia on the…

Gem Notes