This item is very old and has not received maintenance, but nonetheless still appears to work.
General
Richer Asteroids adds non-tiered non-core-fragment ores (coal, gold, diamond etc) to asteroids, while trying to still remain balanced. Do note that this has partly been included in vanilla in the intervening years, but this mod still adds some new ores - eg diamond - and the rates are also somewhat different.
Spawn Rates
Spawn Rates
Vanilla 2016
Vanilla
Modded
Coal
0
0
0.4
Copper
0.65
0.65
0.6
Iron
0
0
0.75
Silver
0
0.45
0.45
Gold
0
0.55
0.55
Diamond
0
0
0.15
Source Code
The source code for Richer Asteroids can be found here:GitHub