can’t make a silk purse out of a sow’s ear

One cannot turn something inherently inferior into something of value. This proverbial metaphor dates from about 1500, and with some slight variation (“silk” is sometimes “vel- vet”) makes its way from proverb collections (by Howell, Ray, Dykes, et al.) into literature (Samuel Richardson, Laurence Sterne, Jonathan Swift, Charles Lamb, Robert Browning, George Bernard Shaw, and Clifford Odets, among others).