It was the broken mouthpiece of a Puebloan tobacco pipe, conically drilled from either end and polished quite round. (That’s a field drawing, with my not-very-big forefinger for scale. Say an inch and some.) Probably argillite, which is “indurated claystone”: claystone that was heated and hardened underground by, for example, a volcanic dike.
I thought of 14th Century men huddled in the lee of the mesa, having a—probably ceremonial—smoke.