The metal icons are rough indicators of a product's sales level. Each level is progressively harder to reach, rather than a simple doubling of the previous level. Levels are (in ascending order):
2643 products (12.2%)
2697 products (12.45%)
1312 products (6.06%)
702 products (3.24%)
278 products (1.28%)
21 products (0.1%)
6 products (0.03%)