The 4Cs are color, clarity, cut, and carat weight.

Gemstones have a wide range of colors. Many minerals are colourless in their pure form and it is the inclusion of chemical impurities in their chemical structures that creates color.

The following is a classification of clarity:
IF (Internally Flawless) Loupe Clean: free of all internal inclusions under 10x magnification loupe
VVS (Very Very Slight Inclusions) Almost Loupe Clean: difficulty to see inclusions under 10x magnification loupe
VS (Very Slight Inclusions) Eye Clean: visible inclusions under 10x magnification loupe
SI (Slight Inclusions) Almost Eye Clean: visible inclusions to naked eye but appearance is not heavly obstructed
I (Included) Visible: obvious inclusions easily seen with naked eye, no need for loupe aid

A gemstone that has a high quality cut will be symmetrical and will reflect light well.

Carat weight
1 carat is 0.2 grams. The density of a gemstone influences the overall weight. This is the reason a diamond, tanzanite, and opal all cut with the exactly the same size and shape will all have different weights.

The specific gravity is the ratio of gemstone density to water density, which is basically the number of times heavier a gemstone of any volume is than water of equal volume. A gemstone with a higher specific gravity will in turn have a higher weight and the lower the specific gravity the lower the weight.
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