Gemmology
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Gemmological equipment and identification
Gemmological equipment and identification

The most used gemmologist’s instrument is a 10 times magnification loupe. Of high quality are the aplanatic and achromatic ones, manufactured specifically for use in gemmology …

Introduction

Popular gemstone — indicolite
Popular gemstone — indicolite, photo © NMNHS

Introduction


This section examines the main characteristics of gemstones such as cut, colour, clarity. It comprises also a number of interesting properties or phenomena possessed by them.

Without trying to give a precise definition of what is a gemstone, we will list some of the things that define it as such.

As gemstones we accept cut specimens of crystals of natural mineral species having a visible transparency. As an exception we can mention the varieties of the chalcedony group, which has a fibrous cryptocrystalline habit but enough transparency or natural opaque minerals, schorl for example. The gemstone must possess a certain durability. Although some collectors think different, we can not accept the inclusion of soft stones with less than 3 on Mohs scale or the other extreme — water soluble.

From over 4800 known minerals (according to data of the International Mineralogical Association), approximately 150 satisfy the above conditions and form crystals of sufficient size to be cut. Currently section ‘Classification’ of our site presents those of which specimens are available — 104. For the rest a brief information exists in the glossary of ‘Library’.

Characteristics

The colour (or the lack of it) is one of the main characteristics of gemstones and perhaps the most important.
The clarity is another important characteristics when evaluating gemstones.
The cut is also of great importance. Generally there are two types of cuts — commercial and precision. The main objective in first one is to preserve as much as possible of the crystal’s size, respectively weight, while the second follows exact proportions, angles, etc., always in relation with the specific mineral that is faceted.
In addition to the above 3 in the evaluation of gemstones an important role plays the size/weight (measured in carats; 1 carat = 200 milligrams).

So-called ‘London blue’ colour variety, obtained by irradiation of colourless topaz — specimen 0070
So-called ‘London blue’ colour variety, obtained by irradiation of colourless topaz — specimen 0070, photo © NMNHS
‘Eye clean’ clarity in danburite — specimen 0502
‘Eye clean’ clarity in danburite — specimen 0502, photo © NMNHS

Treatment in gemstones

Very often natural gemstones are treated to enhance their appearance, colour, or to give them durability.

Phenomena

Some minerals, respectively gemstones, have interesting properties and they can show various optical phenomena such as adularescence, opalescence, pleochroism.