I like the black and white a loooot. The colors of the stripes are all really similar tones and the B&W helps to move away from that and add some varieties
That was my thought with regard to the color as well. Since the colors in that particular stone were all reds, I felt it did not have enough contrast to produce an interesting image. However, I took some today of an amethyst slice that give you more options in terms of color/b&w.
I think with these I am tending to lean towards the colored one, while for the others, the B&W seems to work better.