You have found a chunk of a dark yet reflective ore, lodged deep within the heart of the lode you were mining. The play of light and shadow on its surface is fascinating, almost mesmerizing, and its weight in your hand feels solid, yet not cumbersome. It might be a good idea to bring it to someone familiar with ores and metals, and see what they can tell you about it.
How very interesting... I'm pretty certain I've read about a metal like this, but I'd never actually had a sample in my hand. You wouldn't mind leaving it with me for study, would you? I'd make it worth your time.