Granite Tourmaline - a localized ceremonial material

I have written about granite with tourmaline before [click here] and I was going to show some pictures from the field trip I led this fall to the Manoosnocs west of Leominster, where we saw a lot of it. But those pictures are lost in the innards of a dead computer. Here is an older picture of the material, showing black tourmaline crystals embedded in a matrix of feldspar - a mixture I classify loosely as "granite":
The last site we visited at the Manoosnocs had a great deal of this "granite tourmaline", used in many of the rock piles and always placed in a key location: at the top of a pile, at the corner of a pile, or two pieces placed symmetrically at either side of the middle of the surface of a flat-faced pile, etc. Similarly in the past, when I have found this material used, it always seemed to be the focus of the rock pile's design. As such, in this part of the world, it seems to be used in a way similar to how quartz is often used strategically in rock piles elsewhere: as a key element of the structure.

This material is reasonably common in the glacial till around this area and presumably its ceremonial use is limited to where it was available: down-glacier from a source in the original bedrock to the north of here. I find it used ceremonially in rock piles in a region about like this:
So, mainly it is found in the ~10 square miles south Fitchburg, Leominster, and into Sterling.

Let's think about this some more: How did everyone in this region end up using this same favored material in this same way? Of course there is some quartz available in the same region and also other light, feldspar-rich rocks. But in that case, how did an agreement get made that this was the correct substitute for quartz in this region?

Consider some possibilities: was it one person or a small group of people that were in communication with each other and developed a consensus to use granite tourmaline this way? It seems like too large an area (and too many different sites and piles) for one person to have been solely responsible. Another possibility is that there existed a generalized concept in a culture which automatically took advantage of this material when it was available - and the material's local use was limited exactly by the material's local availability - a local manifestation of a general concept. So if it had been available somewhere else, someone else would have used it without the need for specific consensus via communication.

It is tantalizing to imagine any one of these alternatives and imagine how the specific ceremonial consensus might have been shared. It would take several hours to walk across this region.

Update: I guess another theory is that it might have been started by a single person who was then imitated.