Diamonds are a symbol of the wild, replacing all symbols except for the dispersion - it is also the most valuable symbol, giving you 3000 times the line shares (or 120 times the total shares) on the payline. This feature benefits from the increased multiplier, starting at 2x. The multiplier increases by 1 for every 2 free spins.
This means, for example, that free spins 7 and 8 will be affected by a 5 multiplier, and free spins 9 and 10 will be affected by a 6 multiplier.