It might be it sort of has the shape, like stated above a photo of the edge wouls help. It could be creek or river worn if found around water. like a creek or river.
No. I thought it was a piece of natural quartzite material, but I looked at the pics more and I'll go with the melted glass. It might take longer than there was time for water to produce that sort of erosion on glass. You've got a good preform, maybe see if you could knap it out?!
Thank's for the edge photo, These think's can fool you sometimes it has the outline that would give you the impression of a arrowhead, but the edge is too thick and does not show sign's of edge work it does look old and water worn though. I remember I found a piece of bone one time that had the perfect outline of a arrowhead, but it was just a odd shaped piece of broken bone.