Where did you pick it up at? Knowing the locality helps to identify possible materials. The brown suggests to me a jasper or chert and it appears water worn. Dark blue or blue black *appears black to me* lend it more towards a chert in my opinion.
Does a magnet stick to it? can it be scratched with a knife? can it be scratched with a quartz point? How big is it? how much does it weigh? etc. , etc.