Making perfect stone mortar

[JesseJames]

I'd like to go find a nice basketball sized granite, basalt, or rhyolite cobble and work it into a nice sized portable mortar to eventually try and process acorns with. I'm open to using modern tools to do the bulk of the core removal in order to save time, but willing to do it the traditional way if it yields better results. If anyone has done something similar, I'd love to get your input to help streamline the process and/or potentially avoid a headache.
Another note- I could be off base here, but I've always thought the production of mortars also involved the production of pestles. If that's something to consider, i'd be willing to scrap the power tools and go with pecking/grinding to produce two tools instead of just one. All input welcome. Thanks!

[Kentucky point]

In order to make the process go quicker, choose a round stone with a flat top. I just peck my stuff now-a-days, so I am not sure about modern tools. A chisel might work, but be sure to peck the final touches for an "authentic" look. Pestles are easier to make. I have found most of my pestle rocks in Pennsylvania where the rocks are naturally shaped like that sometimes. In your case, I would have to see the subject stone. I always scrutinize the stone for attack angles.


Here is the one grinding bowl I made.

[JesseJames]

Thanks, I found a rock recently that seemed perfect. If I go back, i'll get a picture before I carry it out. A chisel would work nicely I think. How long do you think the process took you?

[Kentucky point]

I honestly forget how long this took. I think it was like 3-4 hours? I chiseled some of it, and pecked the rest. This rock is from Kentucky - it isn't the best of quality, so therefore it went faster. On the stones that you are talking about, it might take a couple of days.

[JesseJames]

I read a journal from an early 20th century archaeologist studying some mortars found in the ocean in southern california. He tried to reproduce 2 types with basalt(I think) through traditional methods and I think it took him between 12-18 hours to shape the deeper one. Yours looks almost like sandstone, which is what the bedrock mortars are mainly carved out of in ca.