I have a similar object and it was suggested as being a mortar that would be used to grind natural pigments, such as red ochre for paint. Others said it may just be a natural stone. I did find it where I do find genuine stone points. To me the material of which this rock and yours, does suggest to me it was chosen for its abrasive properties. Also I believe the lack of abrasion marks doesn't always mean it wasn't produced by the hand of man. Some minerals will actually dissolve away on their surface by moisture over many years removing all the original abrasion marks that will be barely noticed anymore.