A diamond's size is indicated by its weight, and the weight is indicated in carats: one carat equals 1/5 of a gram. The word carat is often abbreviated as "ct." If a diamond weighs less than one carat, its weight is indicated in points (1/100 of a carat). Therefore, a half-carat stone weighs 50 points, while a 75 point diamond is 3/4 of a carat. If a diamond weighs more than one carat, its weight is expressed in carats and decimals. Therefore, a 1.07 ct. stone weighs one carat and 7 points.
The price of a diamond tends to jump when its carat weight crosses 50 points, 75 points or 1 carat. The price difference between a diamond that weighs 49 points and one that weighs 50 points is often greater than the price difference between one weighing 48 and 49 points. The larger a diamond, the rarer it is. Therefore, as carat weight increases, so does the price per carat. A 2-ct. stone, for example, will be more than twice that of a 1-ct. stone, assuming they are similar in cut, clarity and color.
Diamond color is all about what you can't see. Diamonds are valued by how closely they approach colorlessness – the less color, the higher their value. (The exception to this, of course, is fancy color diamonds, such as pinks and blues, which lie outside this color range.) It may appear that your diamond is colorless, but in fact it probably has tinges of yellow or brown. A diamond's color refers to how noticeable its tints are.
The more noticeable the color, the lower its value, while the less noticeable the color, the rarer it is and the higher its value. There is an exception to this rule: If your diamond has tints of a color other than yellow or brown - for example, if it is blue, purple or red - it is considered to be fancy colored, and is graded differently. Assuming your diamond is in the yellow/brown color range, trained gemologists will grade it based on a finely calibrated scale that is compares it to a master set.
Because diamonds are formed deep within the earth, under extreme heat and pressure, they often contain unique birthmarks, either internal (inclusions) or external (blemishes). Like most things in life, diamonds are rarely perfect. As a diamond forms, tiny crystals may get trapped inside it, or the stone may have irregularities on its surface. Clarity refers to the relative absence of such flaws.
The harder it is to see the flaws, the rarer the diamond. Gemologists will look for two kinds of flaws, or clarity characteristics in your diamond: inclusions, which are flaws enclosed inside, or primarily inside the diamond, and blemishes, which are nicks and scratches on the stone's surface. If the inclusions or blemishes can be seen with the naked eye, the diamond will score the lowest on the clarity scale. The more magnification necessary to see the clarity characteristics, the more valuable the diamond. Don't feel bad if your diamond does not have perfect clarity. Truly flawless diamonds are so rare that many gemologists never see one in their entire careers.
Cut is the factor that fuels a diamond's fire, sparkle and brilliance. An important aspect of the allure of diamonds is the way they sparkle. The sparkle is a result of how a stone is cut - its symmetry, polish, durability, and proportion of its parts - which, in turn, determines how light travels through the stone and back to your eye. Ideally, after light hits the top of a diamond, it travels through the stone and then out the top again, creating the sparkle effect.
This sparkle depends on the proportions between the diamond's table size, crown angle and pavilion depth. Many combinations can achieve the same level of sparkle, but what's most important is how these factors work together in your particular stone to create maximum brightness. For example, if a stone's pavilion is cut too shallow or too deep relative to its crown, light might seep from the side or bottom of the stone, thereby reducing the sparkle. Gemologists will study three light effects in your diamond when determining the quality of its cut: brightness, the combination of all white light reflecting from the stone fire, the diamond's colored flashes and scintillation, and the overall pattern of bright and dark areas when the diamond is moved.