"14-karat" modifies "gold". It states the purity of the gold alloy used. 14-karat gold is 58.5% gold (41.5% other metals). 24-karat gold is 100% gold, but it is rarely used in jewelry as it bends so easily.
So "14-karat gold" modifies necklace: it is not a copper necklace, it is a necklace made of 14-karat gold.
Parla points out that "14-karat-gold" (with 2 dashes) is incorrect.
On websites you may see it shortened to "14K gold", "24K gold" etc.