It may have been something they found while gathering stone, if it is not common in your area, JMO I don't think they used it to grind with. One the Quartz points would break off into what they were working on, I have seen points ,blades, beads and other type items made of Quartz but never a grinding stone with Quartz points on it. JMO the NA like any stone that was bright, shiny and had color and could be worked . I have mined Quartz and it is something about the shine that gets to everyone. Some folks think there is power in the stone and I asked the man that I leased the mine from, did he believe there was power in Quartz and he said yes, it filled his pocket book.
Thanks for the input monster. The way it had been chipped off the other side lead me to believe it would have been used for some sort of abrater/grinder with how hard the crystals are. Pretty much a really course sanding block.
Might of been removed from a stone as a through away due to it not being used. Might of been brought back and never touched but just kept cause of the crystals. They did admire crystals. I have found crystals in camp areas.