My Dearest milum,

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Your pipestone sounds fascinating! Have geologists looked at it? My first guess would be that it is fossilized bone. My next guess is that it is sedimentary rock that was formed around something--like a bone--that held up at the time, but later disintegrated; possibly a fossilized, or partially-fossilized, bone. Just within the limits of believability: the layer of rock was not quite hardened, yet sufficiently firm to hold its shape, when some animal/worm/snake tunneled through it.
If you know of any geologists who have studied this--maybe from a local university?--I'd appreciate it if you'd let me know. I'd like to see if they've published anything on it.