I have no idea what this is. My grandfather found it in some mine tailings near Phoenix, AZ.
I'd agree that it's chalcedony.
I am inclined to agree with Pyrah. It looks like cryptocrystalline quartz (chalcedony, perhaps?)
It does look a lot like chalcedony. I wish I had brought my mineral ID book with me to this apartment. Oh well, c'est la vie.
It kinda looks like quartz to me, but I don't remember forming with swirls like that in it.