A natural rind forms, well, naturally. It develops organically on cheeses that are aged in temperature and humidity controlled rooms. With the passage of time, this rind dries out and forms a hard, protective outer layer.
Edible, but maybe not an experience you'll enjoy. But don't throw it out! Save your natural rinds in a bag in the freezer, and when you're making a soup, broth or sauce, throw that rind in there to cook along with everything else. Fish it out and discard it when you're done cooking and discover a new flavour dimension.