A diamond's value is calculated by a combination of factors: weight, cut, clarity and colour. Each of those factors has to have a grade in order for a specific diamond's value to be calculated. The variance in price for diamonds is staggering depending on the four factors. For example a one carat diamond ranges from a low of £2,470 to £18,850 according to "Jewelry and Gems" by Antoinette L. Marlins. According to diamondse.info a 1.00 to 1.25 carat diamond round cut, fair quality cut, J colour, I1 for clarity is from £1,090 to £1,367. This is a lower quality diamond. The A diamond of the same weight, ideal quality cut, D colour and fl clarity is £18,889. This is a high quality diamond.
Determine the metal used in the setting. Gold is marked with the carat mark. 24 carats (kt) is pure gold. It’s rarely used for jewellery because pure gold is too soft, and bends and scratches easily. The gold is usually mixed with another metal for strength. The other metal determines whether the colour of the gold is white, rose or gold. The proportion of gold to the other metal is designated by the number of carats. 18 kt gold is 18 parts pure gold, 8 parts of another metal. 14 kt gold is half gold, 12 kt gold is half gold, half other metal and finally 10 carat is only 10 parts gold and 14 parts other metal. The higher the percentage of gold the more expensive the setting. Sterling silver is marked and is much less expensive than gold. Platinum is marked as well and is more expensive than gold. Figure out how much precious metal is used in the setting. The more metal by gram weight, the more expensive the ring.
Test the diamond with a diamond tester, which measures the electrical conductivity of the material. Confirm that the stones are diamonds.
Look at the colour of the diamond and request to see any gemological report. Diamonds that are blue-white like crystal clear water are the most valuable. As the colour progresses toward dingy yellow and brown the diamond becomes less valuable. Fancy diamonds or coloured diamonds are in a class by themselves and if natural, are more valuable than white diamonds. Some jewellers give yellow and brown diamonds special names like chocolate diamonds, canary diamonds and cognac diamonds. Black diamonds are seldom used in jewellery and look more like onyx, a flat black stone. The perceived colour of the diamond is influenced by the colour of the background it's viewed against. Black makes yellow less obvious. Look at the diamond against a white background such as a pure white piece of cardboard or paper.
Consider the clarity of the diamond. Nearly all diamonds have flaws. However if the flaw can not be seen under 10X (ten times) magnification, the diamond is classed as flawless. Flaws may be fractures, nicks, inclusions and coloured streaks. The type of flaw and where it is in the diamond affects its value. View the diamond under a jeweller's loupe for any flaws. The more flaws and inclusions, etc., the less valuable the diamond.
Calculate the weight of the diamond by weighing the unset stones or looking at the documentation that the jeweller has on the ring. Diamonds are measured by carats, the more carats the more valuable. Keep in mind that as a diamond gets heavier the price per carat increases exponentially, not arithmetically. In other words a three carat diamond will cost perhaps 20 times what a one carat diamond would cost, all other factors being equal.
The weight of the individual stones is more important to the value than the total weight of all the diamonds. According to diamondse.info in 2010 a three carat diamond nearly flawless and colourless costs £97,500. Six 1/2 carat diamonds of the same quality (a total diamond weight of 3 carats) costs only £7,800.
Factor in the cut of the diamond. Cut refers to how well the diamond is faceted, not the shape it's in, such as round, pear, square or princess. Faceting the diamond perfectly means neither the brilliance (sparkle) or the fire (rainbow effect) were sacrificed for each other.
Trade off factors which aren't important to you for factors that are. If a flaw can't be seen with the naked eye does it really matter to you? A bigger diamond that has a very slight yellowish cast may be preferable over a smaller diamond that's a purer white.
Shop at a reputable jeweller. Diamonds are expensive. It's easy to be fooled into purchasing a diamond that looks bigger because the cut is shallower.