there's a lot of trial and error when choosing hammerstones. One thing they all have in common is durability- you want one that doesn't break. Some people prefer sandstone but not just any sandstone will work. Others prefer harder rocks like quartzite or porphyry. It also depends on what you are knapping. If you are knapping something really tough, a softer hammerstone will often work. For easy materials like obsidian, I use a sandstone for late stages and a harder porphyry for spalling and such.
