WOW!!!!! I think you might have a nice example of pyrite crystals in quartzite (a metamorphic rock).https://en.wikipedia.org/wiki/Cubic_crystal_system
The cube shape is in the Isometric Crystal System. Pyrite, also known as fools gold, often weathers to hematite (Fe oxide). What a nice specimen!! Quartzite is resistant to weathering. The cobble is river rounded and successfully survived being rolled around in the creek. I'd be on the lookout for more interesting rocks. Good observations!! Thank you for sharing your find.