The higher the karat of a gold jewelry is, the less likely it is to tarnish, wherein 24 karat gold cannot tarnish. In general, tarnishing is mostly seen in jewelry that has lower gold karat and is usually seen in items that have less than 14 karats of gold. Little tarnishing may be observed in 14 karat gold jewelries and very seldom in 18 karat gold.
Moreover, in addition to possessing better characteristics, 18K gold is the best investment you can make:
the higher the carat, the higher the pure gold content. The 18K mark indicates that the jewel is 75% pure gold, against 58.3% of the 14K mark.

Gold purity table