One of my favorite items from Viking graves is a whetstone necklace. At least some of them were made of slate. Many don't appear to have the stone type identified, at least in resources I've found so far. However, many are similar in shape. The 3-4" long tapered shape seems to be a normal shape for these utilitarian items. I never have had a really good explanation as to why anyone would go to the effort to shape them just so, but I assumed there was some practical reason I didn't know.
A really good selection of photos can be found on Pinterest at https://www.pinterest.com/cathyr19355/viking-whetstones/ if you want to see the variations in size and shape. The next four photos are taken from there. Note that where stone is identified, it is more often slate than anything else. And there are some examples of beautiful banded slate here.
Still, I have wanted one for years and years. So, when I saw reproduction Viking whetstones available from Ragnar's Ragweed Forge, I was delighted. I now own a jasper whetstone necklace and love it. You can see it in the photo below. It is the red one with a leather thong on it. The surface is quite smooth, so better for final edge polishing than for working out nicks and establishing an edge on a blade.
Some time later, my wife noticed something interesting when we were at a local river. There were a lot of pieces of slate there, many of which made good skipping rocks. However, there were also quite a few with a familiar shape to them!
Is it possible that many or most of the historical Viking whetstones were just found in the shape of a whetstone and used in that shape? Yeah, I think it is possible.
The slate stones are noticeably coarser than the jasper. These river rocks would work fine for establishing the edge of a blade.
I love finding answers, even if only speculative answers, to questions like "why this shape?" Answer: maybe because that is a common shape to find the stones already in.