I picked up several blocks of soapwood in Australia last year. It looks sort of like a cross between tulipwood and canarywood, i.e., lots of variegated colors. I've made only one pen from it, and was surprised how plain it looked. That's only one data point, though.
Apparently "soapwood" is a name given to many different trees and shrubs. I found five different species of soapwood in just Australia in a google search, as well as one in Guyana and the US.
The soapwood I have doesn't have any apparent characteristics of soap, in that it is not slippery nor does it have any strong odor. The tree possibly got its name from some characteristic like the leaves becoming slippery when wet.
Joe