Diamonds facts

Diamonds are formed deep within the Earth: between 100 km and 200 km below the earth's surface. Diamonds form under remarkable conditions.

The temperatures they are formed at are about 900 - 1300 C in this part of the Earth's mantle where diamonds form. The pressure is between 45 - 60 kilo bars. (kB) 50 kB = 150 km or 90 miles below the surface 60 kB = 200 km or 120 miles below the surface.

Diamonds are carried to the surface by volcanic eruptions. The volcanic magma conduit is known as a kimberlite pipe or diamond pipe. We find diamonds as inclusions in the (rather ordinary looking) volcanic rock known as kimberlite. The kimberlite magmas that carry diamonds to the surface are often much younger than the diamonds they transport (the kimberlite magma simply acts as a conveyer belt). To ensure they are not converted to graphite, diamonds must be transported extremely rapidly to the Earth's surface. It is probable that kimberlite lavas carrying diamonds erupt at between 10 and 30 km/hour (Eggler, 1989). Within the last few kilometers, the eruption velocity probably increases to several hundred km/hr.

All natural diamonds are at least 990,000,000 years old. Many are 3,200,000,000 years old (3.2 billion years) how do we know this? Age: from Carbon dating? No! C-dating only works for very young carbon. You need to use other radioactive decay schemes (e.g., uranium-lead) to date in inclusions in diamonds. Inclusions used for dating are around 100 microns in diameter (0.1 mm).

Diamond is the hardest material.

Diamond is the hardest gem on the Mohs hardness scale and graphite (also made from carbon atoms) is the softest! The rating of a mineral's "Hardness" or resistance to being scratched can be given using Mohs' scale. This was devised by the German geologist Frierich Mohs (1773-1839).

Rating Type mineral Everyday equivalent
1 Talc Baby powder
2 Gypsum fingernail
3 Calcite bronze coin
4 Fluorite iron nail
5 Apatite glass
6 Feldspar penknife blader
7 Quartz steel knife
8 Topaz sandpaper
9 Corundum  
10 Diamond  
Most Common Diamond Shapes

Given that both diamond and graphite are made of carbon, this may seem surprising. The explanation is found in the fact that in diamond the carbon atoms are linked together into a three-dimensional network whereas in graphite, the carbon atoms are linked into sheets with very little to hold the sheets together (thus the sheets slide past each other easily, making a very soft material).

How rare are diamonds? How many grams do you need to mine to get 5 grams of diamonds? (5g/1000 kg) @ 1000 g/kg = 5 g /1,000,000 g! But only 20 % are gem quality (80 % of these are sold in a "managed selling environment") and the remainder are used for industrial purposes (this material is known as "bort" or "carbonado" (carbonado is finer)).

Basic Data Hardness = 10
Crystal System = cubic
Refractive Index = 2.42
Dispersion = 0.044
Specific Gravity = 3.52

Treatments and synthetics

Fracture filling of cracks and removal of inclusions. Surface cracks fractures and cleavages reaching the surface can be filled with a glass-like material with similar RI.. Identification: when examined with an optical microscope, filled stones will show: greasy appearance, flash effects, bubbles.

Problem: Filling does not always resist polishing, heating, cleaning, age wear and tare Drilling of inclusions involves using a laser to drill into the inclusion. Solutions can be poured into the resulting "hair-width" diameter hole to bleach colored inclusions. This is compared to getting a filling in your tooth.

Irradiation is used to change the color of the diamond. A common color produced by irradiation are greens yellow and blues, usually very intense and not natural looking.

Synthetic diamonds are often yellowish in color (rarely used for gem purposes, more commonly used as diamond grit for industrial purposes. Modern synthesis of thin film diamond has other industrial applications).

A 5 mm diamond (0.5 carat) takes over a week to grow. Synthesis requires:

  • high pressure
  • high temperature
  • a special apparatus

Synthetic diamonds can sometimes be distinguished from natural diamonds by the presence of flux inclusions (Ni, Al or Fe)

Simulants - That try to simulate the appearance of diamond

The distinction between a synthetic diamond (man-made diamond consisting of carbon atoms arranged in the typical diamond structure) and a diamond simulant (not a carbon compound with the diamond structure) is very important!

In order of increasing R.I., the most common simulates are:

  • YAG = yttrium aluminum garnet
  • GGG = gadolinium gallium garnet
  • CZ = cubic zirconia
  • Strontium titanate
  • Diamond.

Another diamond stimulant is synthetic moissanite (Silicon carbide or carborundum) it was introduced to the jewelry market in 1998 it is one of the better simulates to date, but displays double refraction and may turn yellow under the heat of a low flame.

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