Silver ore is a natural material that can be obtained from Mining a natural silver source block. Each full block contains 1200 ore that can then be washed in an ore washer to have chance of receiving Silver nugget. Becarful not to use "dig tunnel" when mining ore, while it is faster, it will only provide you with 50% of the ore for each action.
The chance of recieving a nugget changes exponentially with the ores quality i.e. 100QL ore will have a 100% chance of washing out a nugget.