This is the rock name to remember when you find a hard, nondescript rock that looks like it could be slate but doesn't have slate's trademark cleavage. Argillite is a low-grade metamorphosed claystone that was subjected to mild heat and pressure without strong directionality. Argillite does have a glamorous side that slate can't match: it is also known as pipestone when it lends itself to carving. The American Indians favored it for pipes and other small ceremonial or decorative objects. You might consider it for your next set of napkin rings. See a specimen in the Metamorphic Rock Types gallery.
Argillite Geology Guide photo