Remember that huge, stone head I carved a while ago? Well the above photos are the 'before and after' shots of how I trimmed the excess off the one-ton block, in about 15 minutes.
If you have a massive block of stone which is too hard to hand-saw, then you need a set of plugs and feathers. I bought my (one inch diameter) set about 30 years ago, and paid a lot of money for them, but they have paid for themselves many times over, both in monetary terms and aesthetics, even though they are rarely used.
Mode de Emploi (excuse my French): Simply decide on where you want the clean break of the stone to be, and score a line to mark it. Drill a series of holes (slightly under one inch diameter) at regular intervals on three sides of the block, on the line. Insert the three-part plugs and feathers, with the wedge-shaped part expanding in the direction of either side of the line. Gently tap the tops of the 'feathers' (an old English term for a wedge is a 'feather') uniformly on all three sides. Each feather will take about three sets of taps before the desired result, and the result is shocking.
GOLDEN RULE: Make sure that you do not have your toes beneath the part of the block which is going to fall without any more warning than a slight change of ring-tone, and make sure that you are standing to the side, so that when the piece falls and rolls onto it's side, both of your legs are not broken, leaving you trapped and calling for help which may never come.
That's it. I do not know the amount of pressure that these little, iron plugs exert, but it is measured in tons, not pounds. Get cracking.