This is a small mod AaronG85 modified based on the work of Claymore.
This changes the boulders (ResourceStones) in game to give you a small chance at mining: Coal, Potassium Nitrate Powder, Lead, Sand and Clay
This also gives you a rare chance of mining: Brass, Silver, Gold and Diamond
Also gives the ability to mine a small amount of iron from terrestrial stone boulders.
The percentages can be changed higher or lower to suit your gaming needs via the blocks.xml.
Updated for A20.
The forum topic of the mod is here.