GTP Any size stone that contains enough material can produce a head. It just depends on your skill level and the quality if the material.. Jasper is usually heat treated to make it easier to knap. I did a lot of practicing using the bottoms of beer bottle and old pieces of glass. Just make sure you are outdoors any time you are knapping. I have no clue on jasper prices but a 5 gallon bucket of heat treated flint ridge cand go from 35-75 dollars. and thats without shipping. If you have access to jasper and bunches of it why not use it. I drive to flint ridge which is three hours from my house just to get some mojo and inspiration from the history of the area. The quarries are now national land marks and can not be mined. You may find the same in your area. If you need some start up help let me know and I can direct you to some stuff on the internet. Here are two of my glass heads.