Jewel__A_Celebration_of_Earth_s_Treasures

016 INTRODUCTION

When the need arises to identify or value a gem, it is sent to

a professional gemologist who is certified by a professional

gemological body. The gemologist will examine the gemstone

for various physical and optical properties in order to make

an identification and evaluate its quality.

An essential quality of a gemstone is durability. A gemstone’s

physical properties determine how durable it is, and how

susceptible it is to wear, breakage, and deterioration, as well

as the quality of its colour. Note that gems with good cleavage

(see right) can be very hard but still be susceptible to cracking.

MOHS SCALE

One of the principal determinants of

durability is the gem’s hardness, or the

relative ease or difficulty with which it can

be scratched. Measured on the Mohs

scale, hardness does not equate to

strength, as very hard minerals can also

be quite brittle. Gemstones below 5 on

the scale are too soft for wear and even

stones of 6 or 7 will scratch and abrade.

Physical properties

Talc

Gypsum

Apatite Fluorite

Orthoclase

Quartz

Topaz

Corundum (sapphire)

Calcite

Very easily scratched

by a fingernail

Can be scratched

by a fingernail

Cannot be scratched by a

fingernail; can be very easily

scratched with a knife

Easily scratched

with a knife but not

as easily as calcite

Scratched with

a knife with

difficulty

Cannot be scratched

with a knife; scratches

glass with difficulty

Scratches

glass easily

Scratches glass

very easily

Cuts glass

Hardness

1 2 3 4 5 6 7 8 9

10

0 1000 2000 3000 4000 5000

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