I remember finding stones similar to this on the shores of Leech Lake in northern Minnesota when I was a child. It took 70 years for me to now discover that this is an omarolluk.
Omarolluks, sometimes shortened to omars, are a distinctive type of glacial erratic that consists of dark siliceous greywacke and exhibits prominent rounded, often deep, hemispherical voids and pits. The hemispherical voids and pits result from the selective dissolution of carbonate concretions within the greywacke...Their rounded shape, whether found in glacial tills or glacial-fluvial (outwash) gravels, indicate that they were eroded from pre-existing littoral or fluvial deposits. Omars are typically found associated with granules and pebbles of oolitic jasper that were transported from the Belcher Islands in Hudson Bay, Canada...... named after Omarolluk, an Inuit man who accompanied and guided R. J. Flaherty on numerous geological surveys of the Belcher Islands and elsewhere in the Canadian north.
With a tip of the hat to the readers of the whatsthisrock subreddit.