not really a "grain" as grains go - there are layers of "color" in the matrix. If you look at all sides of the piece you should be able to see lines on at least two sides of the piece, if you break parallel to the lines ou should get color. If you break perpendicular to the lines you may be able to see the lines in the piece depending on the degree of silica in the lava flow. Higher silica means a clearer piece of obsidian. But you will not usually be able to see any color. The color is a result of light reflecting off of the different mineral "layers" in the matrix.
I do not have time today... I will try to find some good pieces tomorrow and get some pics up.....