"Well, now there’s a mod for that."
The following is unofficial/fan-made content.
Diamonds are a custom enemy based on Roto-Discs, but that follows a linear path. They are used as an example of Custom Timers on this wiki.
In-Game[]

The enemy simply "orbits" the block four tiles below its spawn point, or above, in the case of the reversed versions. While technically not resistant to fire, they will simply not collide with fireballs, so they are fully immune. It should be noted that, due to its linear movement, its distance from the orbited block is smaller at the corners.
How to Use[]
Place all the enemies in the enemies folder in your mappack and select if from the enemies tab in the editor.
Strategies[]
- More than one Diamond can orbit a tile if the reversed versions are used.
- Not all Diamonds orbiting a block must go in the same direction.
- They can be used as a safer alternative to Firebars.