This specimen is a large limb bone from a dinosaur from the Cretaceous of Alberta, Canada. The phosphatic bone material is generally dark in crossed nichols. Most of the canals are infilled with blocky calcite crystals, but a few are still open. The laminar structure of the bone around the Haversian canals is visible. At this scale, the type and shape of the crystals infilling the Haversian canals can be clearly seen.
Arrows point to the infilled Haversian canals.
This is caused by the radial orientation of very tiny hydroxyapatite crystals. When at an intermediate orientation, the crystals cause some light to get through, hence the light zones. Better examples of this effect are shown below for quartz. Note the blocky shape to the crystals, and slightly smaller grains along the inner surface of the Haversian canal. This is due to the initial random nucleation of many crystals on the open surface, and dominance of only some crystals as crystal growth continued.