There is one problem with the pestle theory... there is no wear!! If it was a pestle that had seen use it would be much smoother all around the large end from grinding against the mortar.
It may have have been a fishing weight, net weight, anchor etc. as it looks to me exactly like stone plummets I have seen found in North America.
I don't know a lot about your area, but if my memory is correct there were also stone clubs that looked kind of like that, though I think they were commonly made of greenstone.
Recently I found these, a pecked and ground granite axe and a pecked granite maul. They both have a groove all the way around meaning they are the oldest type. They were made very much like your artifact, by taking a harder tennis ball sized of flint or quartz and hitting it against the granite, each time you hit it a piece the size of a b.b. comes out, that's why they call it pecking. The axes blade was finished by grinding on a wet slab of sandstone.
Yours doesn't show any signs of being ground or of wear at all, just pecking. It is also possible that it is unfinished, but darned if it doesn't look like a very large fishing plummet.
Here are 3 photos of fishing plummets I pulled off of Google Images: