Carat unit of measurement

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What is a carat?
Carat is the unit of measurement used to describe the weight of diamonds. One carat is equal to 0.2 grams, or 0.007 ounces (avoirdupois). Five carats equals 1 gram, and 141.7 carats equals 1 ounce.

Carat weight is commonly expressed in points or fractions. There are 100 points in 1 carat. For instance, 0.33 carats is generally expressed as 33 points, or 1/3 of a carat.
Why is carat weight important?

The carat weight of a diamond is a major factor in its price, but the cut, color, and clarity will also affect the price greatly. A 0.50-carat diamond with high color and clarity ratings may cost more than a 0.75-carat diamond with lower color and clarity ratings.

Within a specific price range, you will find a number of combinations of diamond carat weight and quality. If you value size over quality, you might choose a 1.00-carat diamond with an I or J color and SI2 clarity with a Good cut to get maximum size. For the same price, if you value quality over size, you might choose a 0.75-carat diamond with an E or F color and VS2 clarity with an Ideal cut.

Because large diamonds are much rarer in nature than small ones, diamond value increases exponentially for certain thresholds of diamond carat weights. A 1.00-carat diamond will cost more than twice as much as a 0.50-carat diamond of comparable quality.
How does carat weight affect diamond size?

As diamond carat size increases, both the diameter and the depth of the diamond increase. This is why a 1.00-carat diamond (approximate 6.5mm diameter) does not look twice as wide as a 0.50-carat diamond (approximate 5mm diameter).

When choosing a diamond size, keep the size of the wearer in mind. A 1.00-carat solitaire will appear much larger on a size 4 finger on than a size 8.