A lead chisel is best used in conjunction with a heavy mallet. A flint hammer used with a lead chisel has a chance of breaking the hammer.
Note: This should not be confused with the Wide Tungsten Chisel, which is used in blacksmithing (on an anvil) and cannot be used to gather limestone.
Note: Lead chisels break too, but i didnt count how long it can be used properly. Definately they last longer than the flint ones. (Nathea)
Note: You only get 3 limestone when using both a lead chisel and a heavy mallet. (Coranos)