This is my attempt at measuring the speed of each stone using the in-game speedometer. Keep in mind that it is likely that I made some mistakes. Be that as it may, these measurements should be relatively close. Also I'm not done measuring the various speeds yet but I'm lazy so I'll submit this as it is and update it in the future.

MAXIMUM FALLING SPEED:

Stone: 89 Bouncy Ball: 70 Iron Ball: 83 Jewel: 113 Wooden Fish: 94 Watermelon: 89 Mutagen: 134 Egg: 134 Plate: 108

MINIMUM FALLING SPEED:

Stone: 74 Bouncy Ball: 55