For centuries, the diamond has been desired and coveted. While sapphires, rubies, emeralds, and other coloured gems can paint a piece of jewellery in vibrant colours, it is the diamond that sparkles with fire, adding brilliance to any piece. The diamond has long been a symbol of privilege, and is one of the most appreciated gems the world over.

Properties
Diamonds are essentially made of carbon, and are formed when carbon deposits in the earth are exposed to extremely high temperature and pressure for an extended duration. They are formed between 100 and 200 kilometres below the earth's continental crust in what is known as the graphite-diamond equilibrium boundary. Diamonds that are formed below the earth's oceanic crust are formed at even greater depths due to the lower temperatures.

The diamond is the hardest naturally occurring mineral on earth and can cut any other mineral or gem. It is composed of several allotropes of carbon, including graphite, but what differentiates it from graphite is its chemical and molecular structure, because of which a diamond is so hard. The term 'diamond' itself means 'invincible' and is derived from the Greek word 'adamas'.

However, while a diamond is the hardest gem, it is only of moderate toughness. Toughness refers to the resistance of the gem to stress, which determines whether or not it will fracture easily. A diamond's toughness is moderate because it can fracture more easily along cleavage planes than gems such as the sapphire.

The 4 Cs of a Diamond
Cut: The cut of a diamond, not to be confused with its shape, is an important factor in determining its value. 'Cut' refers to the faceting of a diamond and is influenced by the shape of the rough diamond, location and amount of inclusions, and the final shape desired. A diamond faceted in the 'brilliant' style may be of either ideal cut, shallow cut, or deep cut.

The cut of a diamond determines its brilliance and fire, as it strongly influences reflection of light from the diamond. In an ideal cut, most of the light which enters the diamond is reflected perfectly off its facets back into the observer's eye. A diamond that is of ideal cut is likely to have high brilliance. In a cut which is too shallow or too deep, less light is reflected from the flat top (or table) of the diamond. The faceting of a diamond in a shallow or deep cut allows for light to leave the diamond from the sides or the bottom as well as the table, thereby reducing the amount of light reflected from its table.

The Gemological Institute of America (GIA) five-tier scale for grading a diamond's cut is widely accepted in the industry and ranges from excellent, to poor.

Three of the most important characteristics of a polished cut diamond that reflect its cut grading are brilliance, fire, and scintillation.

Colour: Diamonds may either be clear or may come in various colours, which are known as fancy colour diamonds. Among clear diamonds too there may be some with a slightly yellow tinge caused by nitrogen impurities in the gem. The intensity of the yellow tinge of the stone is directly proportional to the number of nitrogen atoms it contains.

While ascertaining the colour of a diamond, it is imperative to look at the stone from its side and not from the top (table), as the reflection of light from the table may influence the colour perceived. The GIA follows an alphabetic scale to determine the colour of clear diamonds, which ranges from D (an entirely transparent or colourless diamond) to Z (a diamond with a noticeable light yellow or brown tinge).

Fancy coloured diamonds do not fall within this scale, and have been discussed in more detail below.

Clarity: As is the case with most gems that are naturally formed, diamonds may contain flaws or inclusions, either within the stone or on its surface. A diamond's clarity is graded based on the size, number, position, nature and relief of the inclusions it contains.

The GIA scale for diamond clarity designates diamonds as flawless (F), internally flawless (IF), very very slightly included (VVS1 and VVS2), very slightly included (VS1 and VS2), slightly included (SI1 and SI2), or included (I1, I2 and I3). Inclusions in a diamond may be either internal or external. External inclusions are also known as blemishes.

Some of the most common inclusions are:

• Crystal: A three dimensional mineral contained within a diamond.
• Pinpoint: A very small crystal that appears like a dot at 10x magnification.
• Needle: A thin and short crystal that looks like a rod.
• Cloud: Several pinpoints lying together across various planes.
• Feather: a break in a gemstone that extends from the surface to the interior, often white and feathery in appearance.
• Twinning Wisps: A series of pinpoints, clouds or crystals that forms along a diamond's growth plane.
• Cavity: An angular opening created when part of a feather breaks away.
• Indented Natural: A portion of the diamond's rough or original surface that dips below the polished surface.
• Knot: A crystal that extends to the surface.

Some of the most common blemishes are:

• Nicks, Scratches and Abrasions
• Natural: A portion of the diamond's rough or original surface left on a polished stone, usually at the girdle.
• Polish Lines: Fine, transparent or white parallel grooves and ridges left by polishing.

Carat: A "carat" is a unit of mass and represents the size and weight of the diamond. The term is derived from the Greek word keration, meaning "seed of the carob", which was used in ancient times to balance scales. One carat is equivalent to 0.2 grams or 100 points.

When one piece of jewellery has multiple diamonds in it, the total mass of all the diamonds or gemstones in the diamond is known as the "total carat weight."