How to Buy a Diamond
The cut of a diamond isn't actually the size or shape of the diamond, but rather the angles and proportions of the stone. It is the only one of the four Cs that's not determined by nature and is the most important quality to consider. If the diamond is cut too deep or too shallow, it will leak light on the sides, giving it a lackluster appearance, which will reduce its value and brilliance. A well-cut diamond will have two factors: mirror-like facets and a “white light" reflected both internally and externally, also known as the brilliance, or what will make your engagement ring shine bright.
A structurally perfect white diamond has no hue. The degree of a diamond's colorlessness is graded on a scale from D (colorless) to Z (light yellow), with colorless stones being the most valuable. As a general rule of thumb, a colorless diamond will cost your more than a colored stone, unless the color is extremely rare (think: green or red diamonds).
The less impurities or inclusions a diamond has, the clearer and more expensive it is. Inclusions occur when the diamond forms in the earth's mantle, as it was crystallized under heat and pressure, so while there are treatments that boast creating a clearer stone, a “flawless" diamond is extremely rare. Similar to color, the clarity of a diamond is also measured on a scale. Diamonds that are graded in the SI1 and SI2 range are slightly included but their imperfections are often invisible to the naked eye.
Carats refer to the weight of the diamond. As the carat size increases, so does its rarity and, of course, its price tag. But a diamond that has a bigger carat weight doesn't necessarily mean it will look bigger. A diamond with a smaller carat size but a shallow cut may look larger than a diamond with a bigger carat size and a deeper cut. The appearance of size all depends on not only the shape of the diamond, but the different designs and mountings you choose—a halo will make a smaller stone appear larger, and so will a shape with more surface area, like a pear or emerald.