Stargate Atlantis (McShep)
Rodney McKay, while no rock tumbling knucklehead, is aware of the properties of various metals. It can be essential to circuit repair.
What few attempts Rodney had made to purchase jewellery (always for a girlfriend and always truly disastrous) had demonstrated that this knowledge was not useful in a commercial setting. His aesthetic appreciation for such decorations had always been minimal, usually there was warm skin underneath which was far more interesting.
Therefore, he deduced, it was the correlation between John’s skin and the metal of his dog tags that caused him to find the glimmer of silver visually appealing.