By definition, emeralds are gem-quality specimens of the beryl mineral family with a rich, distinctly green color. Because of that, it is inappropriate to use the name “emerald” when marketing a beryl of any other color.

The Federal Trade Commission publishes a set of Guides for the Jewelry, Precious Metals and Pewter Industries. They use “yellow emerald” as an example of an incorrect name that when used in marketing can be “unfair”, “misleading” and “deceptive” (the words here in quotes are straight from FTC guidance for jewelers).

If you are going to buy a “yellow emerald” it might be a good idea to compare it with an equivalent material that is properly marketed as heliodor or yellow beryl. Heliodor is a beautiful gem. It sells for a lot less than emerald and it usually does not suffer from the durability and clarity problems that are common in emeralds.