Powdered emeralds were a supposed cure for the plague that was used during the Renaissance period. The idea behind this treatment was that emeralds were believed to possess certain healing properties. By grinding them into a fine powder, they could be ingested or applied topically to help cure the disease. During the Renaissance, it was believed that the emerald powder would help purify the body and expel toxins. There is no scientific evidence to support the use of emeralds as a cure for any disease, and the practice was likely ineffective. In fact, ingesting or applying emerald powder could have been harmful, as emeralds are a form of beryllium and can be toxic if ingested in large quantities.