This is just a guess but it looks to me like a chunk of quartz (the white areas) with something else veining. Quartz is usually safe (many aquarium gravels are made from quartz). You can test it with an acid and if it fizzes it is calcareous which means it will slowly dissolve calcium into the water, raising the hardness and pH. Vinegar is one acid, though some say it is too weak to really test, but it usually works well enough. If you have the API nitrate test kit, you could use a drop or two of Regent #2 which is a stronger acid.
Wash it well in very hot (but never boiling) water. Use an old toothbrush or similar to scrub it.