A diamond is a naturally found pure solid form of diamond which is measured in terms of the unit CARAT. This generally measures the weight of the diamond rather than measuring the size of it. More the carat, the heavier the diamond will be. 

A carat of diamond equals 0.2 grams. The size and dimensions can be a good indicator for its value. For example a round cut diamond of 6.5mm would be of approximately 1 carat. 

There are certain things one should remember while purchasing a ring regarding the carat. 

- One should consider both, cut and carat while making a purchase. A larger carat with a poor cut can have the appearance of a smaller diamond as compared to the diamond of the same size with much higher cut. 

- The ring size should be importantly considered before buying a ring. This makes a huge impact on the carat of the ring. The carat size should be according to the ring size as the ring might look large or small on the finger. 

- Setting style should also be considered as the setting style for each ring is different. The stones set in a delicate setting, appear a bit larger than they are than in a thicker setting. 

These are some important details that one needs to remember about while making a purchase regarding the cut and carat of the diamond. 

The shape of the diamond can also make a difference in the decision about the carat. As some shapes can make some amount of carat appear larger, while the same amount of carat can look smaller in the other shape. For example, the marquise and pear shaped ones can appear larger, as their shape is already elongated.

One should always set a budget and go according to that as well. Try not to go overboard with your budget. For example, a diamond of 0.90 carat diamond costs much less than a diamond of 1.00 carat. And by look it is not easy to distinguish between the two. It can only be made out in the grading certificate or by a professional jeweler through special tools. It is advisable for a person to ask the jeweler for a carat guide chart so that one can make a choice according to the size of the stone in reference to the carat of the gemstone. As every shape has a different size in different settings and carat.

For example:- 

In case of pear shape rings:- 

Carat weight        mm

0.25                  5.5 x 3.6

0.50                  6.5 x 4.6

0.75                  7.6 x 5

1.00                  8.3 x 5.2

1.25                  9.50 x 6.10

1.50                  9.5 x 6.2

2.00                  10.5 x 7

2.50                  12.00 x 8.60

And another shape mm will differ in the same carat weight. For instance, In the case of round shape diamonds-

Carat weight        mm
0.25                      4
0.50                     5.2
0.75                     5.8
1.00                     6.5
1.25                     7.0
1.50                     7.2
2.00                     8.0

As seen in the charts above, a different carat weight has a different size. Therefore, there should be surety about the size and the carat weight. It should be correctly matched and made sure of while making a purchase. 

The carat weight is very important and one of the 4Cs of the diamonds which should be made sure of while purchasing a diamond.