Yoma 44b-45a says there are seven types of gold: gold, good gold, Ophir gold, Mufaz gold, Shachut gold, Sagur good, and Paryavam gold, and proceeds to explain what they are.
Let’s say that Reuven sells Shimon a gold alloy and claims that it’s gold. Shimon gets it checked by a metallurgist and comes back to Reuven, saying that he thought it was pure gold. Is this considered a mistaken sale, for which Reuven must return the proceeds of the sale?
On the one hand, Reuven said “gold,” and in common parlance, that means pure gold (with some degree of error on its purity), and that’s not what he sold.
On the other hand, the Gemara says that there are different types of gold, many of which seem to be alloys (ex. Mufaz gold, according to Rashi, sounds like white gold; Paryavam gold, defined as red gold, is a gold-copper alloy; etc.). Does this definition of gold (i.e. any of these seven gold-based metals) extend to common sales, and if Shimon meant that he wanted pure gold, he must explicitly say so?