Gemology Guide - Physical Properties of Gemstones

In the first chapter of SDI’s Gemology Guide we dealt with the cardinal virtues of a gemstone, now moving further we will explore the nature of these colorful and vibrant gems. In other words we will get to know about the properties of gemstones. The basic properties of gems have an extremely important bearing for not only who wish to study gemology but also for gem cutters, buyers and for people who deal in the gemstone markets. Properties of gemstones can be broadly classified into two: physical  and optical. Physical properties are determined most importantly by the crystal structure and chemical composition of a gemstone whereas the optical properties are dependent on various other factors along with the crystal structure.

This post will talk about the physical properties of a gemstone.

First and foremost, to have a crystal clear understanding of a gem’s physical properties it is required to be acquainted with what is a crystal structure of a gem. Each and every gem whether organic or inorganic is made up of atoms. These atoms are the basic building blocks for any gem. Now, almost all the gems are  inorganic minerals and these minerals always grow in a specific order termed as the crystalline structure. The regular, repeating arrangement of the atoms determine the crystal structure. The symmetry of the arrangement of atoms defines the outer shape of the gem crystal.  The crystal structure affects various properties of a gem like hardness, cleavage, fracture, specific gravity etc.

AMETHYST CRYSTALS

There are certain organic gems like opal and amber which are also made up of atoms but these atoms lack a systematic arrangement. Hence these gems are called amorphous gems.

OPAL-AMORPHOUS GEM

Certain factors like heat, pressure, space for crystal growth, chemicals available in the growing environment, cooling time etc, hugely affect the growth of crystals. These are basic requirements for gem crystals to grow. In any case if these requirements are not met it can lead to distortion in the growth. The atoms can get arranged in random patterns, when this happens aggregate gems are formed.

JADE-AGGREGATE GEM

Classic example of this is Amethyst which belongs to Quartz group and chalcedony which also belongs to the same group. Amethyst is a well formed crystal of the quartz group whereas chalcedony is an aggregate gem which results from disturbance which is caused  in the growth process of a crystal.

There are seven basic crystal systems in which the gem crystals grow.

Cubic – eg. Diamond, spinel, garnet etc.

Tetragonal – eg. Zircon

Hexagonal – eg. Berly and apatite

Trigonal – eg. Corundum, quartz, tourmaline

Orthorhombic – eg. Topaz, iolite, tanzanite, chrysoberyl, peridot

Monolinic – eg. Kunzite spodumene, moonstone orthoclase feldspar

Triclinc – eg. Amazonite, rhodonite,turquoise.

PHYSICAL PROPERTIES

Hardness: one of the most important virtue of a gemstone is its durability. But what makes a gem durable? Hardness of a gem is what makes it durable. Hardness can be defined as the ability of a gem to resist scratches. This property is greatly dependent on the crystal structure and how strong is the bond between the atoms. Hardness is usually expressed through Mohs Scale wherein various minerals are listed and given a number depending on its ability to resist scratches. This scale is based on the concept that a hard mineral can scratch another softer mineral. For example quartz can be easily scratched by corundum.

MOHS SCALE

(top to bottom – hard to soft)

10. Diamond

 9. Corundum

 8. Topaz

 7. Quartz

 6. Orthoclase Feldspar

 5. Apatite

 4. Fluorite

 3. Calcite

 2. Gypsum

 1. Talc

Toughness/Tenacity: This property is often confused with hardness but these are two different concepts altogether. Toughness is the ability of gemstone to withstand breaking and chipping. In other words how well can a gem take physical stress. Aggregate stones like jade and nephrite are excellent examples of tough stones. Topaz on the other hand is a very brittle stone.

Cleavage: it is a characteristic manner in which a gem breaks when an external force or stress is applied to it. Usually the break comes parallel to the area wherein the bond between the atoms is weak. This break creates smooth flat surfaces along the plane of atomic weakness.

CLEAVAGE

Parting: for one to understand what is parting in a gem, it is required to understand the concept of twinning. Sometimes during the growth of a crystal, the internal arrangement of atoms is disturbed. This disturbance is caused by certain changes in the growing atmosphere of the crystal. This leads to a change in the growth direction of the crystal forming twins. This process is termed as twinning and the location where the direction of growth changes it is called as twinning plane.

Twinning might take place not only when the crystals are growing but even after that due to metamorphic pressure.

When a gem breaks in a direction which is parallel to the twinning plane, it is termed as parting.

 

CONTACT TWINNING

CYCLIC TWINNING

LAMELLAR TWINNING

PENETRATION TWINNING

Fracture: besides cleavage and parting, any other break in a gem is called a fracture. Most transparent gems break in a typical pattern creating circular and concave ridges. This is called conchoidal fracture. There are other kinds of fractures as well splintery, granular, sub-conchoidal etc.

 

CONCHOIDAL FRACTURE

Specific Gravity: it is the measurement of density of a mineral. Two minerals of the same size  might weigh differently due to the variation in the density of minerals. Specific gravity can be described as a ratio between the weight of the mineral in air to the difference of  the weight of mineral in air and mineral in water.

In other words SG= wt. of mineral in air/ wt. of mineral in air – wt. of     mineral in water.

SG is a very important tool in gem identification.

Properties of gemstones is an imperative subject of gemology. It is a vital aspect which not only aids one in understanding the very nature of gems but also is of significant help to gemstone facet-ors. Along with these, it also gives crucial hints which can help one take good care of their gems.

*The Author is a Colored Stone Graduate from Gemological Institute of America, Mumbai and a Diamond Graduate from Solitaire Diamond Institute, Bangalore. She also holds a Diploma in Jewelry Designing & Manufacturing from Jewelry Product Development Centre, Jaipur. She also blogs at 'The Jewel Affair'.

*Picture Courtesy: Google Images