PURE (100%) GOLD
Pure gold is too soft, and is usually mixed with other metal alloys (silver, copper, nickel and zinc) to make it stronger and more durable for jewelry.
18K GOLD / 14K GOLD
18k gold contains a higher (75%) percentage of pure gold and therefore is more valuable than 14k gold (58.3%). A marking such as ’14k’ or ’18k’ indicates the karat, and you may sometimes also see the manufacturers registered trademark or the country of origin.
YELLOW, WHITE, & ROSE GOLD
Pure gold (which is always yellow) is too soft for making jewelry. The metal alloys that are mixed with pure gold for strength can also modify the resulting color to produce different shades of yellow, white, and pink gold. White gold was originally developed to imitate platinum, and usually contains 25% nickel and zinc or palladium. Nickel is cheaper than palladium and therefore more widely White gold is also rhodium plated to make the white gold whiter.
A side note about Rhodium: It is also a precious metal yet not feasible to make solid jewelry. Believe it or not it is about ten times if not more costlier than gold but it is a great for plating white gold jewelry, it is much more whiter, chrome like and makes a piece dazzling. It can also be used on silver, preventing it from quickly tarnishing. Rhodium plating will also make diamonds look bigger and brighter, nothing sets off diamonds like rhodium plating.
There are many other factors that determine the value of a gold jewelry piece: Weight - gold is sold by weight - grams(gr) or pennyweights(pwt), the heavier the piece, the higher the gold content, therefore it is more expensive.
Design - designer jewelry is more expensive, especially if it is a one of a kind piece.
Finish - special finishes to the metal such as matte, or sand-blasted finish add to the cost of the gold jewelry piece.