Parma Violets were a mostly uk phenomenon, manufactured by the Derbyshire-based company Swizzels Matlow.
The sweets are hard, biconcave disc-shaped sweets, similar to the Fizzers product from the same company, but without their fizziness. There was also a line of Giant Parma Violets.
Most people found these sweets less than tasty, but had an allegiance to them that cannot be fathomed. They swarmed into the collective consciousness and have stayed there ever since. Now-a-days there are shops, bands, books and specialist brands that all use Parma Violets in their product/ businesses.