The American comedian Groucho Marx famously said, "Outside of a dog, a book is a man's best friend. Inside of a dog it's too dark to read." Humor aside, no one would take this to mean one specific book or dog. Same with emeralds. Whereas gold is used as a mass noun, emerald is not. (In fact, "a gold" would be wrong, unless gold is used as an adjective.) Therefore, Sentence 1 sounds wrong. (At least to my New York ear.) Thus, "emerald" must be used as a count noun, as in Sentence 2, or the plural version given by Tushar Raj in the comment.
I will allow that it may sound different in other countries.