Category: Gem Notes

A Sapphire’s Secret

INTRODUCTION At Lotus Gemology, our Bangkok-based gem testing laboratory, we only test for two types of stones: corundum (ruby & sapphire) and spinel. As a result, we often get parcels of stones with similar attributes. This…

Chrysoberyl

Introduction Chrysoberyl, BeAl2O4, is a mineral species with well-known varieties, namely alexandrite and chrysoberyl cat’s eye. Much has been written about chrysoberyl’s varieties, so this gem note is intended to focus on the transparent faceted material. Chrysoberyl…

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…

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…

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…

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…

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…

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…

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…

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…