I picked this Molcajete up at a thrift store. It is large and very heavy. I have tried curing it with with dry rice and soft rice a few times. It may have already been cured.
I don't think it is concrete. The pestle has a big black mass/spot you can see in the images. It also has some sparkly spots in the light.
What I am worried about is when I scrape the bowl, it leaves scrape marks. You can see I made some in one of the pics (maybe 1st pic, but not sure how they will load). This is my first one so is that normal? If it is normal, is it cured properly?