A metalloid. A not quite metallic enough metal. They sure *look* like metal.... except when you drop it and it shatters like glass! Antimony would be one good example of this (though it's not quite as fragile as glass). Perhaps the most interesting anecdote about antimony is that its chief ore, a mineral known as stibnite, was crushed in ancient Egypt to be used as eyeliner. Today its uses are far less glamorous: medicine, industrial chemistry applications and in fireproofing.

Anyway, here's a crystalline piece of it embedded in Lucite, the clearest of all acrylic resins. 99.999%+ pure.