Once the clarity is SI2 or above so no inclusions can be seen by the naked eye and the inclusions do not affect the fire from a nicely cut stone, which in the high street is very rare.
Colour is the predominant factor. Colour attracts the human eye this is very obvious in the fact that when you are in a room with friend of in a public bar and a television is on you will always look at it you cannot help it because of the moving colour images. It does not matter what is on the television you will always look try not to. I will always look.
The very best grades of diamonds are colourless the only way to explain the colour is the colour of iced water. But in my opinion and of quite a few experts this colour is not always the best.
In the smaller stones I would buy a G or H colour because the stone will reflect the colour of the mounting it is in so in reality I would not buy a D colour in any size less than a 48 point unless you want the best colour available. Which I think is fine if you want the best and you can afford the best buy the best.
Without having a set of master stones you would have to be very good at grading stones to be able to say what colour the stone is also you would have to know a lot about fluorescence and be able to recognise it in different lights. This can make a D colour look like the worst coloured stone you have seen for a long time in a dirty tinged yellow fluorescent light.
If you presented me with a low D colour and a high E colour I would need a set of master stones to be able to grade the 2 colours.
The high street goes on about the 4cs but they are just selling a commodity. Have you really talked to anyone in a high street chain who knows what he or she is talking about I haven't if you don't buy from me see a specialist jeweller who knows what he is talking about and you will end up with a much better stone. I deal with quite a few specialist jewellers. If you would like a recommendation I am happy to recommend any of my trade customers and I will guarantee that you will purchase a much better diamond. If you learn more in the future you will still be happy with your choice.
Back to the colour of diamonds the absorption of light determines the colour of diamonds and the trace element in the stone decide the different colours. The main trace elements in coloured diamonds are chromium, copper, magnesium, vanadium, titanium, nickel, cobalt and iron.
When grading a diamond you look through the sides of the diamond to grade the colour and place a master stones either side to determine the correct colour. This must be done under set lighting conditions.
Pink diamonds do not contain these trace elements their colour is created by high temperature and natural radiation as they were formed. They are the rarest diamonds in the world only 40 or so intense pink diamonds over 75 points are found every year and they all come from the Argyle mine in Australia. They have just found another deposit in Australia near to the Argyle mine and as yet they have not found any pink but they will in small numbers.
Colours were first categorised in the United States in the 1930s the country with the biggest diamond market. The colours were named after diamond deposits and mines. The top colour was Jager that was a very clear diamond with a Very light blue cast. These diamonds were named after the Jagerfontein mine in South Africa.
This was later found to be fluorescence in the stone and the second colour was moved to the top. The second colour was called river this was a very clear diamond the colour of iced water it was called this because most of the best diamonds were found in rivers and alluvial deposits. It is now D colour.
Today grading starts at D leaving A B C in reserve in case a clearer colour is ever found.
The GIA grading scale is as follows. D E F colourless. G H I J near colourless. K L M faint yellow. N O P Q R very light yellow. S T U V W X light yellow. Y Z fancy yellow. Y colour is on the border and can be either light yellow or fancy yellow this is down to the individual grader as all stones are still graded by eye. If you have a particular question please e mail me firstname.lastname@example.org
There is a lot more to colour than I have written here but these are I feel are the main points without going to deeply into the science of colour.