Appears like I'm always getting calls from folks asking "How can I crack a geode open having to break it into small pieces?" Well, there are a variety of ways, some good and a few not so good. Here, I will outline four of the most common ways. [By the way, it's wise to use safety goggles whenever you start banging away in a rock.]
1) Diamond Saw - For anyone who is lucky enough to have a large diamond saw, or know someone that owns one, you are able to saw the geodes in two. This is most effective if you find cigerettes to carry the specimen. You are able to cut the geode open and end up with two halves having a smooth face on each. (But not everyone has diamond jewelry saw....)
2) Another method of opening geodes that works well, (the method I personally use most often) is to crack the geode open by having an old fashioned cast-iron plumbing pipe cutter. This is a tool that plumbers accustomed to use in doing plumbing in homes once they worked with the surefire plumbing pipes. (Homes built prior to about 35 years ago.) I have one of these simple tools that I use to break geodes open and most of times I can break them with two matching halves. This does indeed a great job.
Perhaps you have seen show dealers that were breaking geodes open using these tools. The tool includes a chain at the business end with links just like a bicycle chain, and in this chain there are round carbide rollers with sharp edges on them.
To break the geode open, you simply wrap the chain, with the carbide rollers, round the geode, and fasten it into a notch within the oral appliance press recorded on the handle. This constricts the chain round the geode evenly all around and squeezes to where it breaks the geode open into two halves. I received my cast iron plumbing pipe cutter from a retired plumber here locally. If you're likely to break lots of geodes you might want to acquire one.
3) Hammer and chisel. Most people posess zero diamond saw or plumbing pipe cutter, and just wish to break a few geodes open. Well, you can do an excellent job with a hammer and cold chisel. I did it a number of times and it works well. Take your hammer, cold chisel and geodes outside high is really a concrete walk, driveway, steps, etc., in other words a hard surface. I would not test the fit a wood surface. And you almost need three hands to get this done. Put the geode around the concrete and hold it around the sides with one hand, then contain the cold chisel on top of the geode and strike it LIGHTLY together with your hammer. Don't try to break it open now.... Rotate the geode about a half inch, put the chisel consistent with in which you just hit it and strike the chisel again ... LIGHTLY. Do this all around the biggest area of the geode. By the time you have hit the chisel in a line all the way around the geode, it should be ready to open. If the geode hasn't broken open at this point, start round the circumference again, striking the geode using the cold chisel in a straight line. Strike the chisel just a little harder this time. This can be a little slow, however if you simply do it right and do not enter too big a hurry, you should be in a position to break the geode open into two halves that you can fit back together to where you cannot tell where it had been broken.
4) Hammer - Obviously you can strike the geode repeatedly with the hammer until it breaks open, but, it probably go ahead a few pieces. Not a good method!