If Sonic and friends were ponies.
||Power Sneakers (power-up), because even though he doesn't have his usual feet, his shoe is widely reconised as a symbol of his own speed.
||Spanner, symbol for engineering.
||Fighting Glove, represent his strength.
||Piko Hammer, Her signature spells are hammer-spawn, heavy object levitation and teleporting to wherever Sonic is.
||Hover Shoe, his own version of Sonic's cutie mark.
||Crystal, for her love of gems.
||None, diamond dogs don't have cutie marks. I figured being bigger than ponies and use of fore-arms to build stuff.
||Psychokinesis symbol. In Sonic '06 there were marked out locations where Silver was able to use his powers to create paths from rubble, the symbols for these marks can be his cutie mark. Also Silver specializes in levitation spell.