Gorgeous ring!
Many, many stones are "used". Since they are thousands and thousands of years old, though, they shouldn't be thought of as such. Many jewelers buy previously owned stones and often have them re-certified before reselling. When purchasing a diamond that hasn't been recently cut its impossible to know how many previous owners there might be. But like sufergirljen said, it's not like a used car. A diamond's value is not less because it wasn't recently cut.
In Tiffany's case, they take the diamonds out of the settings, inspect them and reset them into new settings.
Are they aloud to sell a stone without disclosing that it was used?
Many, many stones are "used". Since they are thousands and thousands of years old, though, they shouldn't be thought of as such. Many jewelers buy previously owned stones and often have them re-certified before reselling. When purchasing a diamond that hasn't been recently cut its impossible to know how many previous owners there might be. But like sufergirljen said, it's not like a used car. A diamond's value is not less because it wasn't recently cut.
In Tiffany's case, they take the diamonds out of the settings, inspect them and reset them into new settings.