Along with that hammer, you will need chisels to extract that precious crystal out of that hard rock. Just like hammers they come in many shapes and sizes,for example, the Gad Point, is a chisel with 4 sided point. They are useful to punch a hole in rock and get a crack started, but they are not useful in extraction. The best chisels are designed for masonry work and are used by stone masons and especially hardened for splitting rock. The only draw back is they are expensive. The cheap bargain cold chisels, bought at most stores are next to useless, the point is hardened too much and chances are it will probably break the first time you use it.
If you are a veteran collector you will probably have numerous chisels of varying sizes and know how to use them. To the beginners out there, here is a hint on what you need for basic chisels. I suggest you get several 1/2 inch diameter chisels of various lengths, maybe 6, 10 or 12 inches long. You will also need several 1 inch chisels in the 8 and 12 inch lengths and one or two small ones 1/4 inch by 6 inch and maybe even a foot long one for delicate crystal extraction in deeper cavities. The thinner chisels cannot be pounded on with any great force as they will bend. Chisels shaped like an L are not very useful. If you pound on a chisel with a sledge you generally need at least a 1 inch chisel or thicker or they will bend.
I myself have chisels made out of Rebar, the steel they use to reinforce concrete. It is a high carbon steel and makes great long lasting chisel, but you will need to find a blacksmith to fashion them into chisels. They work great because they are ribbed and are easy to hang unto. The Estwing company makes some useful chisels with rubber guards attached, that offers some protection for your hands. I find that your tools are easier to spot if you paint them a bright colour; I use fluorescent orange, they might look gaudy but it does help keeping track of my tools, so buy yourself a can of bright paint and become a tool steel graffity artist.
When I go collecting at Herkimer I probably take several dozen chisels. I also take along sharpened car and truck springs: they are used like chisels and because they are thinner they are very useful in cracks. When I head for Herkimer between the hammers,chisels , springs, bars and wedges, I might have two hundred pounds of steel. I even take an Angle grinder with me to sharpen chisels and grind down the burrs that will mushroom out on the butt end. When a dedicated rock collector thinks he has all the chisels he will ever need, he will find out he is just starting and will go to great lengths to improve upon his tools. He will start looking at flea markets, auctions , yard sales and mineral shows for good steel. The hunt for chisels will become as interesting as the hunting for the crystals.
I always take a large selection of chisels with me when I go to Herkimer: the rock down there is very tough on them. I have had favourite chisels that I have used for years, but alas , just like me it only takes a few hours of hard pounding and were done for the day.