Hackmanite

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 hackmanite is better characterized …

Clinohumite

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 amounts of hydroxyl and …

Kornerupine

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 were distinct species with …

Andalusite

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 pressure. Andalusite is primarily mined …

Diopside

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 species but along with hedenbergite, …

Prehnite

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 used as a gem …

Apatite

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 called by its species …

Fluorite

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 of jewelry. It does …

Benitoite

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 material is in California. Although …

Tugtupite

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 north coast of the Tunugdliarfik …

Ethiopian Opal

Since reemerging in the market after a decade long hiatus, opal from Ethiopia has generated a lot of buzz during the past six years. Anyone visiting a major gem show since 2009 has likely seen, quite literally, piles of Ethiopian …

Vesuvianite

Introduction: Vesuvianite is a tetragonal mineral with the formula Ca10Mg2Al4(SiO4)5(Si2O7)2(OH)4. As the name implies, it was named after the locality of its discovery, Mount Vesuvius, Campania, Italy. It has an interesting history in the nomenclature process. Originally discovered in the …

Sphalerite

Introduction: Sphalerite is zinc sulfide (ZnS), also known as zinc blende. It is a fairly common mineral crystallizing in the isometric system and is a principal ore of zinc. In addition, it often contains iron with minor amounts of manganese …

Gem Note: Hauyne

Introduction: Haüyne is an extremey rare mineral, a member of the sodalite group, and is even more rare as a gemstone with the formula (Na,Ca)4-8Al6Si6(O,S)24(SO4,CL)1-2. It is sometimes seen in the literature as hauyne or hauynite, both synonyms for the …

Serendibite

Introduction: Serendibite is an extremely rare mineral with the chemical formula Ca2(Mg,Al)6(Si,Al,B)6O20. The mineral was first discovered in 1902 by G.T. Prior and A.K. Coomaraswamy at Gangapitiya, which is near Ambakotte, approximately 12 miles (20 km) east of Kandy, Sri …