DIAMOND ABRASIVE GRINDING WHEELS.
At least, you're trying to cut ceramic fired materials.
When you cut a magnet in half, you end up taking a N S pole device and turning it into a N S and N S device.
THus, you can "ruin" the polarity.
Better yet, get them uncharged, cut them then magnetize them. You should be albe to "set" the polarity when you do it that way, depeding on how the magnet structure is oriented.