Carats are a unit of measurement used for gems.  One carat equals 200 milligrams.  Many people refer to the size of a gem in terms of its carats especially in regards to diamonds.  However gems of the same size don't always weigh the same and gemstones come in different shapes as well.  For the purpose of comparison though listed below is a chart of the different sizes and shapes of diamonds and how many carats they are which can be used to approximate carats for other gems as well.

CARAT SIZE                     MILLIMETER WIDTH

Round shape

0.25.....................................4.1mm

0.50.....................................5.2mm

0.75.....................................5.9mm

1.00.....................................6.5mm

1.25.....................................7.0mm

1.50.....................................7.4mm

2.00.....................................8.2mm

2.50.....................................9.0mm

Oval shape

0.25.....................................5.0 x 3.0mm

0.40.....................................6.0 x 4.0mm

0.75.....................................7.0 x 5.0mm

1.00.....................................7.5 x 5.5mm

1.25.....................................8.0 x 6.0mm

1.50.....................................8.5 x 6.5mm

2.00.....................................9.5 x 7.5mm

3.00.....................................10.0 x 8.0mm

Marquise shape

0.25.....................................5.5 x 2.5mm

0.50.....................................8.0 x 4.0mm

0.75.....................................9.0 x 4.5mm

1.00.....................................10.0 x 4.75mm

1.25.....................................10.5 x 5.0mm

1.50.....................................11.0 x 5.0mm

2.00.....................................12.0 x 6.0mm

3.00.....................................14.0 x 7.0mm

Emerald Shape

0.25.....................................5.0 x 3.0mm

0.50.....................................6.0 x 4.0mm

0.75.....................................6.5 x 4.5mm

1.00.....................................7.0 x 5.0mm

1.50.....................................8.0 x 6.0mm

2.00.....................................8.5 x 6.5mm

3.00.....................................10.0 x 8.0mm

Princess shape

0.25.....................................3.4 x 3.4mm

0.50.....................................4.4 x 4.4mm

1.00.....................................5.5 x 5.5mm

1.50.....................................6.5 x 6.5mm

2.00....................................7.5 x 7.5mm

2.50....................................8.0 x 8.0mm

3.00....................................8.3 x 8.3mm

4.40....................................9.0 x 9.0mm

Pear shape

0.20....................................4.0 x 2.0mm

0.50....................................6.0 x 4.0mm

0.75....................................7.0 x 5.0mm

1.0......................................8.0 x 5.5mm

1.5......................................9.0 x 6.0mm

2.0.....................................10.0 x 6.5mm

2.5.....................................11.0 x 7.0mm

3.0.....................................12.0 x 8.0mm

 Trillion shape

0.25..................................4.0 x 4.0mm

0.50..................................5.0 x 5.0mm

1.00..................................6.5 x 6.5mm

1.50.................................7.0 x 7.0mm

2.00..................................8.0 x 8.0mm

3.00..................................9.0 x 9.0mm

4.00................................10.0 x 10.0mm

5.00................................11.0 x 11.0mm