Drop chances

ItemStage 10Stage 50Stage 100Avg count
Forge Ahead75%75%75%0.75
Small Stone25%25%25%20.24
Oil Shale25%25%25%17.64
Crushed Sand25%25%25%20.07
Clay Soil25%25%25%19.52
Cobblestone Rocks12%12%12%9.49
Concrete Mix12%12%12%5.81
Cement12%12%12%9.32

Average count per open at loot stage 50. From 40,000 simulated opens per stage; perks that raise loot are not counted.