In the Star Trek original series episode "Balance of Terror", there is a briefing room scene. In the scene, Spock holds up a sample of recovered debris. Spock's in-universe description calls it:
From the outpost's protective shield. Cast rodinium. This is the hardest substance known to our science.
From a production perspective, the scene starts with Nimoy holding a prop which has to be sturdy enough to handle and pretty much wave about, yet brittle enough to shatter with an apparently effortless squeeze (although it might not have been entirely effortless).
What was the prop made of?