Karat

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Photo courtesy of Scott Papper, GIA GG, AJP, CM (NAJA), Global Gemology & Appraisals

Karats are units used to measure the fineness of gold.

24K = Pure Gold
18K = 75% Gold
14K = 58.5% Gold
10K = 41.7% Gold
9K = 37.5% Gold

While 14 karat gold is always 58.5% pure gold, its other alloys determine what color the gold will be.  White gold will have a higher percentage of nickel while rose gold will have higher copper content.