It's impossible to tell what kind of wood it is by my untrained eye, particularly as it's not wood at all. It's entirely mineral. Fascinating stuff but not particularly pretty or valuable. I'm guessing it's not valuable, but it may be geologically important. I went back the next morning and found four more pieces in the same general area. It's pretty clear that it was dredged up by the construction. One one of them I found had some extremely fine silt in a crevice, and it looked exactly like volcanic ash I encountered while planting trees in the Mt. St. Helens area after its eruption, so I'm wondering if it could be volcanic ash. When did the last volcano erupt on Taiwan anyway?

Regardless, they are kind of cool in a geeky way. Definitely keepers.