Fatwood is usually resinous conifer such as pine. When a tree is cut, the roots don't know it and continue to send sap up through the roots to the stump. For a while at least. The sap is aromatic, and a good bit of it evaporates, leaving behind the sticky resin/rosen. As the stump ages, the softer, less resionous wood decays. The wood with the higher concentration of sap is much more weather/bug/fungus resistant and remains. This is thesapwood...the fatwood. Most you can buy in stores is imported by a company in North Carolina from their pine farms in Honduras. There is some domestically made, but not a lot. And a lot of what is sold as fatwood is nothing more than pine lumber scrap.
I prefer spruce or cedar when I can get it. I have cedar here on the farm. Some of the rafters in the collapsed barn, like my skinning and tool shed, are over 100 years old. The high resin content makes them resistant to insects like boring carpenter bees and termites. Standing deadwood is good too as the entire resin content is preserved in the sapwood. It just has to be split to get to it. With the weathered stumps, you can kick away the rotted portion and the resinous portions remain stout and standing.
Will it take a spark? Here is a site with some photos using a ferocious rod.
http://www.caliberdt.com/~bill/fatwood/index.htm
Codger