Thread: 8" x 2" stone in box marked 1896 -- what is it?

Wondering if the pink/red/brown top part was discoloration from oil, I decided to soak it in a strong trisodium phosphate solution for a couple of days. Strong enough that I had to neutralize the stone with vinegar afterwards because my fingers were getting soapy and slick just rinsing it off under running water (TSP is basic enough to saponify the oils in your fingers). And... nope, here are the pics after two days in a strong TSP solution:







Looks the same, just as pretty! It's almost but not quite flat. The center along the long axis is humped upwards slightly. I'll flatten it and then start using it -- should be fun!