It is really fossil dinosaur bone and 250 MILLION years in the making. The bone was buried in mud in ancient streams, lakes and volcanic events. Over the eons the calcium has been replaced by other minerals, mostly calacite and agate. The colors are caused by minerals nearthe bone with the reds being caused by iron as an example.
The red bone piece is agate and the brown one initially had some replacement in calacite that was then agatized with some cells being fully agatized. Agate is harder and takes a better polish than the calacite so it is the better material. In the case of the brown one it isthe best of both minerals as the calacite added colors and some refraction not normally seen in agate only replacement.......