CARAT:
The metric carat, which equals 0.20 gram or 1/142 of an ounce, is the standard unit of weight for diamonds and most other gems. One carat is divided into 100 "points." A diamond weighing 50 points = 0.5 carat (ct.), and is expressed as one-half carat. The carat-weight of a diamond is the most obvious characteristic of a diamond, and larger stones are often prized due to their rarity. It must be remembered, however, that the weight has no bearing on quality, and two diamonds of comparable weight may differ drastically in price due to dissimilarities in cut, color, and clarity. Diamonds of the same diameter may vary in actual carat weight depending on variations in their proportions. All other aspects being equal, the larger the carat weight, the more valuable the diamond is.

One Grain or "a one grainer":
25 points or 1/4ct.; A diamond of approximately 1ct. would be called a 4 grainer, one ~ 1.5ct. would be a 6 grainer, and a 0.5ct. stone a 2 grainer.

Melee:
Diamonds weighing less than 1/5ct. (less than 20 points). Stones this size are set using various techniques as groupings in jewelry.

Weight representation:
In the U.S., the Federal Trade Commission requires that diamonds be weighed to one-thousandth of one carat (0.001ct.), but that the final weight can be rounded to the nearest half point (0.005ct.). This means that a diamond weighing 0.995ct. can be represented to you as a 1.00ct. stone. However, international convention is more strict, and a weight can only be rounded to the next higher point (0.01ct.) if the actual weight reaches 9/10ths (0.009ct.) of a point. For example, the 0.995ct. diamond must be represented as a 0.99ct. stone, and to be represented as a 1.00ct. diamond the stone must weight 0.999ct. before it can be rounded up to 1.00ct.

Value per carat:
Increases with carat size, because larger rough diamonds occur less frequently. In other words, 2 half-carat diamonds taken together will not cost as much as 1 one-carat diamond, as the one-carat stone is more rare. Also, a premium is added to diamond prices as they reach and exceed each 1/4ct. increment in weight.

Fluorescence:
Produced by ultraviolet light from the sun, by black lighting or other long-wavelength UV source, occurs in an estimated 35% of gem grade diamonds. (Virtually all diamonds fluoresce when exposed to X-rays, and this forms the basis for their identification and collection at mining sites.) The UV light excites electrons in the diamond crystal, which then release this absorbed energy in the form of visible light, producing a blue, or sometimes other color, of faint to very strong intensity. Once the light source is removed however, the fluorescence is no longer observed. If, in rare situations, light emission continues for a period after the exciting light has been turned off, the phenomenon is called phosphorescence.

Blue Fluorescence:
If strong or very strong, may alter the perceived color of a diamond in a negative or positive way. For example, stones in the colorless/near colorless ranges (D-H) may appear milky or oily, detracting from their appearance. On the other hand, diamonds in the lower, more yellow color ranges (I and lower) may appear to have less yellow color due to the fluorescence, adding positively to their appearance. Thus, the trade will slightly discount prices of diamonds in the former category, while sometimes adding a very slight premium to those in the latter.The presence and color (most frequently blue, but can be any color) of fluorescence and its intensity (none/inert, faint, medium, strong, very strong) are indicated on all GIA and AGS grading reports.

Irradiation:
Of diamonds using a nuclear reactor or linear accelerator, sometimes in combination with heating, is used to produce a variety of different fancy colored stones.

High Pressure/High Temperature (HPHT) treatment:
Is a process developed by General Electric whereby type IIa diamonds of low color (N-O) or even fancy brown color, can be converted to colorless/near colorless (D-H) forms by an annealing process involving high pressure and temperature.There is some concern that these color-enhanced stones may come to market undetected, but new research has shown that they can be largely detected using expensive and sophisticated equipment. Similarly, the HPHT technique has also been applied to type Ia brown diamonds by several companies, yielding fancy yellowish green and greenish yellow colors. Pink and blue colors have also been produced by the same technique.

SHAPE:
In addition to the traditional Round Brilliant shape, the most common Fancy Shapes are: Oval, Pear, Marquise, Princess, Radiant, Emerald and Heart. See the chart below! Less common shapes include Trillion (or Triangular), Flanders and Asscher.

TABLE and DEPTH:
Percentage are two critical measurements for loose diamonds. The angles of cut will affect a diamond's natural ability to both reflect and refract light - that is, the diamond's ability to virtually explode with brilliance. If a diamond is cut too deep, light leaks out the sides, brilliance is lost and the center of the diamond will appear to be dark. If a diamond is cut too shallow, light leaks out the bottom and the diamond appears watery, glassy and dark. If the proportions are good, the sides and bottom of the diamond will all reflect their light back out the top of the diamond, which creates the brilliance diamonds are famous for.

The TABLE percent is the width of the top facet of the crown compared to the diamond's total width. Table width between 53% and 64% are considered Preferred in Round Brilliant Cut diamonds. The DEPTH percent is the total depth or height of the diamond compared to the total width. Depth measurements between 58% and 63% are considered Preferred in Round Brilliant Cut Diamonds. The Preferred range for these measurements will change as the shape changes, i.e. a Marquise cut diamond will have different ranges for Premium Table and Depth percentages.