Seeing how I use a rock hammer and chisel to do my extractions.. I would do the same for the larger areas. Focus on seams or cracks first. I've found that a well placed hit or a couple of hits.. these cracks will loosen and simply fall away, Remember to wear leather gloves and safety glasses.
When you get closer to the crystals themselves, you will need to use a slab saw (water cooled) and slow cut around the area. Don't try to take it all in one swoop. I still have a couple of large Agate vugs I'm awaiting to brake into smaller sizes that I can handle. May get around to do so this spring.