Dirk says first and foremost, look for a stone that is 'alive', has sparkle and lustre. Can you see a black inclusion, or imperfection? Is the stone flat, is the clarity not good and colour down? If you answer yes to any of these questions, then do not buy a stone. Ultimately, there are four factors that determine the value of a diamond, known collectively as the four Cs.
Impurities in a stone block light. Although these inclusions as they are called, make a stone unique, the fewer there are, the more valuable the stone is. The standard measurement starts from flawless (IF or IF) to imperfect (I). To be flawless, a stone must have no inclusion visible to a trained eye, even under 10 times magnification under a good light.
This is the weight of a diamond. The word is derived from the carob seeds that were used to balance scales in the past. A carat is equal to 200 milligrams and there are 142 carats to one ounce. There are about 100 points to one carat. A common weight sold is one and a half carats. All else being equal, diamond price increases with diamond carat weight, because larger diamonds are more rare and more desirable. But two diamonds of equal carat weight can have very different values (and prices) depending on three other factors of the diamond 4Cs: Clarity, Color, and Cut. It's important to remember that a diamond's value is determined using all of the 4Cs, not just carat weight.
Diamonds can come in every colour of the spectrum but the most popular are colourless. Icy white diamonds are extremely rare and therefore the most expensive. Stones are graded by how far they stick to or deviate from the purest white, starting from the grade of D, descending down through the alphabet. Every letter designates a more yellow tint.
Cut refers not to a diamond's shape (e.g. round, oval, pear, etc.) but to a diamond's proportions, symmetry and polish. The beauty of a diamond depends more on cut than any other factor. The creation of the Diamond 4Cs meant two very important things: diamond quality could be communicated in a universal language, and diamond customers could now know exactly what they were about to purchase.