Hornblende is an unremarkable dark gray stone which can be found in metamorphic and both igneous stone layers.
In Real Life
Hornblende is typically a component of igneous rocks such as granite, basalt, or gabbro. It often contains titanium, which the dwarves do not have the technology to refine (no electricity).
Hornblende can be processed into white asbestos. Unfortunately dwarven strand extractors can't do that.