Diamond Physical Properties

education in the diamond industry :

evaluation of diamond quality :

the first C, of the 4 C's carat weight :

the round brilliant cut :

hearts and arrows :

the third c of the 4 C's clarity :

the forth c of the 4 C's colour :

an overview of coloured gemstones :

identification of diamond and simulants :

diamond manufacturing process :

precautions during manufacturing :

weight estimations :

role of laboratories in the diamond industry :

the latest technology in the diamond industry :

laser processing in diamond manufacturing :

the global diamond industry as a whole :

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major diamond mining countries :

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world famous diamonds :

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Physical Properties of Diamonds

Composition

Diamonds are formed by nature at high pressure by process of crystallization deep in the earth. Diamond is carbon in its most concentrated form. Except for trace impurities like boron and nitrogen, diamond is composed solely of carbon, the chemical element that is fundamental to all life, cut diamond is distinctly different from its close cousins, the common mineral graphite and Lonsdaleite, both of which are also composed of carbon. Why is diamond the hardest surface known while graphite is exceedingly soft? Why is diamond transparent while graphite is opaque and metallic black? What is it that makes diamond so unique? A diamond is formed by the process of crystallization. The act or process by which a substance in solidifying assumes the form and structure of a crystal, or becomes crystallized; the formation of crystals. The key to these questions lie in diamond's particular arrangement of carbon atoms or its crystal structure. The feature that defines any mineral's fundamental properties. A crystal is a solid body formed from the bonding of atomic elements or compounds in a repeating arrangement. Often, crystals possess smooth external faces. Due to their symmetrical and finite nature, the building blocks of crystals are limited to relatively small numbers of atoms, and their chemical compositions to simple numerical combinations of elements

Lonsdaleite Structure

Figure 5.1 :
This model shows how each carbon atom (ball) is connected to 4 other carbon atoms by strong
chemical bonds (rods), creating diamond's rigid crystal structure.

Figure 5.2 : These idealized drawings show some of the common crystal habits of diamond.
Clockwise from left to right they are an octahedron, a cuba-octahedron (a combined form),
a dodecahedron, a macle twin, and a cube.

Diamond on Mohs' Relative Mineral Hardness Scale

The Mohs' scale was devised by Friedrich Mohs' in early 19th century and has been a valuable aid to identifying minerals ever since. Here are the ten standard minerals in the Mohs' scale.

1. Talc

2. Gypsum

3. Calcite

4. Fluorite

5. Apatite

6. Feldspar

7. Quartz

8. Topaz

9. Corundum

10. Diamond

You use the Mohs' scale by testing your unknown mineral against one of these standard minerals. Whichever one scratches the other is harder, and if both scratch each other they are both having the same hardness. The Mohs' scale is strictly a relative scale, but that's all that anyone needs. In terms of absolute hardness diamond (hardness 10) actually is 4 times harder than corundum (hardness 9,) and 6 times. harder than topaz (hardness 8). Because it isn't made for that kind of precision, the Mohs scale uses half numbers for in-between hardness. Dolomite, which scratches calcite but not fluorite, has a Mohs' hardness of 3.5. Thus, on Mohs' mineral hardness scale diamond is the most hard mineral. Composite hardness is in proportion to the constituent. Two well known methods used are the Mohs scale and Rosiwall's scale. However, these techniques, although popular with geologists are imprecise and are not suitable for determination of abrasiveness.

A comparative study among Mohs' Hardness, Simple Hardness and Rosiwall's Cutting Hardness of different gems each gem and mineral has different scale of hardness.

Moh's Scale Image

A Substance harder than diamond

Boron Nitrate it is harder than diamond, But it is not a gem quality stone, Its optical properties are not like a gem so it of much less value. The hardness of diamond is 10 but the hardness of Boron Nitrate is 10.5.

Reason of inclusions in a diamond

Dark included crystal or a black spot is non converted carbon. Due to variation of temperature the black spot remained unconverted into pure diamond. Sometimes some other element is found in diamond which looks like a black spot this is also known as natts or naturals.

Cutting hardness of diamond

Diamonds cutting hardness (Rosiwal ) number is 140,000. Corundum the element below has the index of 1,000. Indentation hardness of diamond ranges between 6,900 and 15,000 kg\sq.mm, corresponding hardness of the corundum being 1350 and 2,000 kg.\sq. mm.

Factors responsible for the durability of diamond

The three factors responsible for the durability of a diamond are:

1. Hardness :
The resistance power to scratching in a diamond is called the Hardness of a diamond. A diamond can only be scratched by a diamond. Tight atomic lattice gives diamond its unique hardness.

2. Toughness :
The power of a diamond to resist crack or fracture is called its Toughness. Nephrite Jade is tougher than diamond, but diamond is not as brittle as glass that is why most of diamonds are polished without breakage.

3. Stability :
The ability to remain stable in the presence of excess heat and severe cold is known as the Stability of a diamond. No change in colour or size occurs after acid boiling. But the latest formulae of HPHT treatments and irradiation treatments can change the colour diamonds.

All of the these three factors determine the durability of diamonds. These special qualities of hardness, toughness and stability make diamonds unique.

The Chemical equation of Diamond

The chemical equation for diamond is : 2FeS + C02 = 2FeO + S2 + C = (Diamond)

Note:
Other theories use different values for temperature and pressure to some extent and as a result, the chemical equation can also be partially or completely different according to the alternate theories.

The effect of water and grease on diamond

Diamond behaves differently with water if the concentrate which has been mixed with water is allowed to run across the conveyor covered with a layer of grease. The so called GREASE TABLE which is an incline and shock mechanism , the diamonds adhere to the grease and remain in place whilst rubbish material runs off with the water. The diamonds then only need to be scraped off the conveyor and the grease can then be boiled off.

General Physical Properties of Diamond

General physical properties of diamond
Property	Value	Units
Hardness	10,000	kg/mm2
Strength, tensile	>1.2	Gpa
Strength, compressive	>110	Gpa
Fracture strength	400-800	MPa at < 1 mm
Density	3.52	g/cm3
Thermal conductivity	10-20	W/cm-K
Coefficient of friction	0.05 (dry)	Dimensionless
Optical index of refraction	(at (591 nm) 2.41	Dimensionless
Optical absorption coefficient	0.05-0.3 at 10	microns
Resistivity	1013 - 1016	Ohm-cm

What is Graphite

Graphite is the low allotrope of Carbon. It is a soft, black substance which occurs naturally. The structure of graphite consists of layered hexagonal rings of SP-2 hybridized carbon atoms. These layers are held by weak Van der Waals type forces resulting from the interactions between clouds of delocalised P electrons from each of the layers. Graphite is also known as black lead and when mixed with clay, forms the "Lead" in pencils. In its natural form it is used as a dry lubricant and its electrical conductivity makes it popular as an electrode material.

L- Solid Graphite and R- the Molecular Structure of Graphite
Natural Graphite Solid

Tension in Diamonds

Tension of a diamond is also known as 'stress'. Tension can be identified using polariscope through which light appears iridescent inside diamond. Natural Characteristics of individual diamond related to internal structure are visible or non visible inclusions. Presence of tension can cause diamond to shatter.

As viewed by a Polariscope - Tension in a diamond:
Tension in Diamond

Trigon in diamond

Trigons are formed on octahedral faces. The points of the trigons always face dodecahedral faces, while their bases face the cube faces. Trigons are believed to be the result of natural etching or erosion.

This is an image of Trigons was created with Normarski differential interface contrast microscopy and is 0.29mm across. (GIA)
Trigon in diamond

Diamonds are developed under immense pressure

At a depth of 100 km below the Earths surface and in consideration to the dimension of 25.4 square mm or 1 square inch the pressure is about 45 ton' (0.1 million pounds). The crystal of diamond is developed at 150 to 300 km below the Earths surface, at 300 km below the surface of the earth, the weight of the earth is close to 135 ton per 25.4sq mm (0.3 million pounds). You can imagine how much pressure is required for the diamond crystal to be developed.

Popular methods of diamond treatment

The most wide spread methods of diamond treatment are irradiation, thermic processing, coating, fracture filling cracks and laser drilling for removing inclusions. These are all artificial methods changing the appearance or properties of a gem stone to improve its sale quality.