Molybdenum

Molybdenum was a precious metallic chemical element. It had a melting point of 2622 degrees centigrade.

The Sense Sphere was rich in molybdenum. In the 28th century, two separate Earth expeditions discovered this and wanted to mine the planet for it. The Sensorites held the second expedition prisoner to protect their world from being exploited. (DW: The Sensorites)