My guess is that the edge is black painted.What Rivem said. Looking at Edmund Optics I see two lens substrates that match color wise the lens you have. One is germanium and the other is silicon.
OP was a troll and banned accordingly, so no need.My guess is that the edge is black painted.
Germanium and silicon - due to a high refractive index, IR lenses are simpler, and mostly singlets. Though, who knows what is that without seeing through. :can:
OP could tell if a lens transmits light. What is a focal length. Can even try to measure radii.