It is formed by low temperature - high pressure alteration of olivine (lherzolite-dunite) and olivine rich basalt usually deep within the subduction zone where one tectonic plate dives under another at a continental margin.

My experience is that serpentinite rock splits where it wants. The combination of being relatively soft, having no internal bedding structure, and having formed under very high pressure makes breaking random, and usually not where you want.

The rock is a mix of lots of things, including magnetite, clinochlore, garnets, etc. The colors do not necessarily indicate anything in particular.

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Regarding the tan coating of serpentine rocks that I mentioned above, I've been scouring the web for it, and found this image, in which the upper  third of the rocks have that same color coating. Maybe just different kind of rock, though. I've found more samples of serpentine-ish rock in Santa Rosa creek beds, small through huge in size, all with this sort of coating. The question of what this coating is, has really got my attention. I cracked open a small sample and noticed that the tan material is also inside as thin coatings or veins. But not the classic parallel strands of  chrysotile, possibly another configuration, and possibly weathered. Can you connect me with anyone who might share this interest, to figure out what that material is? My next step is to scrape a bit of it off and look at it under 40x - 100x microscope and see if it has a stranded nature. Thanks - Paul