The Difference Between Vermeil, Plated, Filled & Solid Gold
Gold plated jewelry is made of a base metal like brass that has been dipped in a solid gold bath, in such a way that the gold content is less than 1%. While they may have a similar luster and appearance at first, this layer quickly fades when washed, rinsed, or rubbed too hard, leaving behind discolored skin and faded jewelry. Keep in mind that most fashion jewelry is gold-plated, which explains the lower price point.
Gold Vermeil is a common type of gold plating, which uses sterling silver as the base metal. Vermeil is more hypoallergenic and has a thicker layer of gold than normal gold plating. However, the plating still wears off with enough scuffs and scratches.
By law, gold filled jewelry must contain 5% gold, so instead of being dipped in gold, Gold Filled jewelry undergoes a mechanical bonding process that melts a thicker layer of gold onto the base metal. As such, the inside of a gold-filled jewelry piece will still be stamped with a karat number, however this is only applicable to the filled coating. While filled jewelry will maintain its gold cast for longer, it will inevitably undergo discoloration and tarnishing after time.
When jewelers simply refer to "gold," they are usually talking about solid gold. While it's the highest quality of gold, it isn't 100% gold element referred to as Au on the periodic table. In its most pure form, 24k gold is soft and altogether too fluid to work with. Because of this, alloys are added to create a more structurally sound metal known as "solid gold."
Not all solid gold has equal proportions of pure gold, however. 14k solid gold is 14 parts pure gold and 10 parts other alloys, whereas 18k gold is 18 parts gold, 6 parts alloys. You can learn more through this link.