Both ‘karat’ and ‘carat’ are used as units of measure and purity however -

 Karat: Is the measure of the purity of gold where 24kt (karat) is purest meaning 99.9% is pure gold. For example:

 - Her husband bought her a 22karat gold necklace for their 10th wedding anniversary.

 Carat: It is the measure used for knowing the mass of various gemstones and pearls; 1carat is equivalent to 200miligrams. For diamonds in particular, a stone is considered flawless only if it is of 100carats. For example:

 - Her husband bought her a 5carat diamond ring for their 10th wedding anniversary.