What is Gold Vermeil?

By GovMint :

Gold Vermeil, pronounced "ver-may," is a type of high-quality jewelry that features a thick layer of gold over a base of sterling silver. To qualify as vermeil in the U.S., the gold layer must be at least 2.5 microns thick and have a minimum purity of 10 karats. Unlike standard gold-plated jewelry, which often uses a base of brass or copper and a thinner gold layer, vermeil is characterized by a thicker gold layer and sterling silver base. 

Sources Used:

https://www.usmint.gov/learn/tours-and-locations/fort-knox