SOME IMPORTANT PROPERTIES OF GEMSTONES

Some important properties of gemstones are the things which set apart them

from other ordinary glass like, glittering stones of earth. They are as follows,

Crystal habit, Crystal system, Colour, Hardness, Refractive index, Lustre,

Cleavage, Density, Specific gravity and Dispersion.

Crystal habit.

It can be defined as the unique growth of the particular mineral crystal which

may vary with various growth conditions like temperature and pressure. When

one mineral is replaced by another by preserving the original growth, it is

called as Pseudomorphous replacement. Most of gemstones are not found as

perfectly formed single crystals but are found aggregate of several crystals

grown together in one mass,

The crystal habit is influenced by the following factors as follows,

1. Crystal twinning, 2. Crystal growth conditions ( temperature and pressure)

3. Trace impurities.

For eg. Precious stones like diamonds have their growth in very deep part of

earth at very high pressure and high temperature. Most of the diamonds

are free from metallic impurities and they are having elemental carbon in

crystalline form. But some times they appear in different colours by the nitro-

-gen impurities found in them.

Crystal system

A crystal system is a category of

space groups which characterize

the symmetry of strucure in 3 di-

-mensions. (i. e.) x axis, y axis

and z axis. This is applied to ca-

-tegorize crystals. For eg. diamond which is also called as cubic zirconia is in

cubic system. The unit cell is in cubic shape.

Colour

If all the gemstones were available in colourless form they would not have

reach this much of popularity as they are having in their colourful form. The

different colours are giving special attraction to all variety of gemstones.

For eg. chromium found in Aluminium oxides of corundum, ruby is giving dark

pink colour to them. Emerald is getting it's beautiful colour with the addition of

Barium.

Hardness

This measures the scracth resistance of various minerals through the ability

of harder material to scratch softer material. It was created in 1812 by German

mineralogist Friedrich Mohs. However it is one of the several definitions of

hardness in material science. As we all know that Diamond is the hardest of

all the gemstones with a Mohs scale of 10.

Refractive index

It is a measure of how much the speed of the light is reduced inside the

medium. For eg. refractive index for a glass is 1.5.

It can be obtained by the formula n = c / vp

n = refractive index

c = speed of the lght in air or vaccum

vp = velocity of the light in medium.

The refractive index of glass shows the speed of light in it is 0.67 times the

speed of the same in vaccum or air.

Lustre.

It describe the way of light interacts with the surface of a crystal. For eg. dia-

-mond is said to have adamantine lustre and pyrite is having metallic lustre.

Cleavage

This is the tendency of crystalline materials to split along definite planes

creating smooth surfaces.

For eg. Basal cleavage - parallel to the base of the crystal.

Cubic cleavage - parallel to the faces of cube.

Diagonal cleavage - parallel to a diagonal plane.

Density

Density is the mass per unit volume. The ratio of the mass of the material to

it's volume. For eg. an iron ball is heavier than a rubber ball. As the most

gemstones are metallic oxides and silicates they are found to be heaveir than

ordinary stones of earth.

Specific gravity

It is the ratio of the density of the material to the density of water.

Dispersion

It means the phase velocity of light on it's frequency.

v = c / n

c = velocity of light in air or vaccum.

n = refractive index.

v = phase velocity of the light.

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