Some of what I found out in an earlier thread---
Helps to recognize the grain (flow?) It does not always go straight and is susceptible to 180's.
The flow also seems to have a (lap) component. Smooth flake from one side of the bi face chatter from the opposite side. Sort of like banded agate. Also I found flakes to actually hump over the more white veins. Very tough (the white). I can be rewarding though when you get yourself a keeper.
Zuma