I would say the many of this type bead are found in places other than graves, I would not be surprised if the majority are found either in stashes or separately as surface/near surface finds.

Here is an example within about five miles of where Suzi and I live one of the local ranchers while out on horseback spotted an old pottery vessel laying on top of the ground, when he investigated, the pot was from the Mimbres culture and to his amazement it was full of turquoise beads. There is now a large copper mine in the area and it is known for the quality turquoise that has been unearthed, needless to say the Mimbres Indians that lived around here would have utilized this beautiful stone and there would have been those that were bead makers, perhaps this pot was a storage vessel owned by one of the bead makers.

This said don’t think all of the wonderful stone beads we deal in are from graves again I would think that the majority are not. IMHO

All my best ....... Danny

P.S. Over the years I have found more than a few ancient stone beads on anthills even some as surface finds.