Thanks Charl for meeting up with me today to look at my "bowl/mortar"
Your most welcome! Wish we could have been more definitive, but here is what we do know.
The stone is conglomerate. This is a sedimentary rock composed of pebbles all the way up to cobble/boulder size rocks embedded in a sandstone matrix. The stronger the ancient river, the more size to the cobbles swept downstream to eventually become part of a formation of conglomerate, thousands of feet thick, and after the millions of years required. Looking at the second set of photos, at the photo in the top right corner, of the underside of the piece, one can see some of the pebbles embedded in sandstone. There was one good sized hardstone pebble, not sure if it's seen in the photos.
Looking at the topside, I noticed that the texture of the surface of the cavity, hard to see unless in person, was coarse grained. It is sandstone after all. But it was coarse, neither made slick, or at least smoothed, by swirling water, nor did it look like any grinding had taken place as if pecked and ground into shape. So, one possible natural explanation is that a very large cobble eroded out of the matrix. In other words, it might have been that kind of conglomerate, with very large stones included in that sandstone matrix, because large cobbles were deposited in the sand by a very strong river. A formation that was once thousands of feet of conglomerate, and what you have is a tiny piece of that, a piece with one very big cobble having eroded out and left that cavity. Think of the type of cement walls with boulders embedded, and that might convey the idea. Only, in the case of the rock, the bond is millions of years old and much stronger. Also, conglomerate would be a poor choice to use to make a mortar or bowl. The maker, as he hollowed it out, would have kept running into pebbles that were likely hardstone rocks, and he might end up cursing quite a bit
Having said all that, as you know StringFellowHawk, it still was a head scratcher. It is clearly broke, and not recently, at it's thin spot, making it look like half a bowl. Just did not see the grinding I wanted to see in the cavity. I still hope you show it anyway to the folks you mentioned. If they were curious, bring it in. Show it around, mine is just one in person opinion.
Nice meeting you, give me a holler anytime......
Oh, that was the nicest triangle from around here that I've seen in quite awhile. Thanks for bringing it!