What i read is the op was working on his tractor and in the process of cleaning his brake parts he noticed a stain on his "Granite precision table" that is made of "Stone." Unless this is some kinda new/old technology hybrid tractor that utilize the advanced technology of Stone Parts I dont think its a tractor part that has the stain the OP is concerned with.The OP is working on a tractor brake system, not a granite surface plate...
I could and very well may be wrong but thats my interpretation based on the ops words.