|Dinosaur Bone, Jurassic, Colorado, USA|
The distinction can be well seen here in both photographs. The partial weathering away of the smooth external bone has resulted in the exposure of the spongy bone interiors. Geographic context is important, so knowing where it was found is very helpful for an ID.
Knowing the geologic context of your find can help you to figure out if you've perhaps found a terrestrial or marine fossil. Did you find any other fossils nearby?
Can you see pieces of fossil shells or remnants of fossil leaves? Things get tricky with erratics. That's when something has deposited a rock or fossil far from the place it originated. We see this with glaciers. The ice can act like a plough, lifting up and pushing a rock to a new location, then melting away to leave something out of context. If you do think you have found fossil bone, it is likely that your local government would like you to report it. You may have found something very significant. I very much hope you have.