About the Diamond
It rolls when an egg hatches, with a 1% chance. The Mutation Madness admin weather doubles that.
It stacks with one weather trait: with Eternal (x12.5) a pet earns x87.5. On a Dragon (300M/s) that would be 26.3B/s.
Income with Diamond
A pet can hold one hatch mutation and one weather trait at the same time, so they multiply together.
Questions about the Diamond
What does Diamond do in Ride A Pet?
It multiplies a pet's money and speed by 7.
How do you get Diamond?
It has a 1% chance to roll when you hatch an egg.
Numbers from the Ride A Pet community wiki (CC BY-SA); write-up, comparisons and maths by BloxStonks. Pictures belong to the game's developer.