Yoko is the name of a very large 20 pound sledge hammer we use to break up really really big stones. She can break huge chunks of granite and limestone, and any other type of rock we come across in our day to day work of building dry stone walls. My friend Evan Oxland bought this beauty in Ottawa, at Preston's Hardware ( a great place to buy well-crafted masonry tools) He gave her that name in deference to Yoko Ono who was powerful enough to be able to break up the greatest rock group in history.
Yoko does a good job, but like other smaller sledge hammers she still has to be swung carefully. Accuracy, not just strength, is needed to split a big rock the way you want it to break. If it is done right, a large rock will yield two halves. These halves will usually have flat straight faces running at right angles to the length of the original stone. These split stones can then be laid structurally along a wall, with their faces showing out. Most often (at least with the quarried flattish limestone material we usually break up) a stone will break better if it is hit off to the side of the middle. If hit in the very middle, it will break into several pieces that are often too small or too triangular to be useful as builders.
To get a crisp long break it is best to hit the rock near its edge, (but not so near that it just chips a chunk off the side) When hit properly the break will spread in one long straight line across the stone. By contrast if the rock is hit in the middle, it will often break spreading cracks out from the point of impact along several radiating lines, creating half a dozen, less useful, short triangle pieces.