Diamond education: What is a carat? Carat is the unit in which a diamond is weighed. But don’t be fooled – a high carat weight doesn’t necessarily mean a larger diamond. Cut and shape can play a big part in how a diamond appears, so that 2 carat ring on your ringer might appear smaller than a diamond that weighs less. And because finding large diamonds are rare, a single large-carat diamond will cost more than a ring with multiple diamonds that equal the same weight.