1) this ia an old idea. They make hot dog cooks that use an elongated concave piece of polished metal.
2) Also cigarette have seen these as lighter.
3) it is what probable inspired the chocolate used to polish the bottome of a soda can.

Finally have you ever really tried to start a fire with a maganifing glass using natural material. You may get smoke but that is a long way from a flame.

cheers