In which episodes of Star Trek: TOS were Dilithium Crystals shown on screen? This is not counting episodes where they were just mentioned; only episodes where they were actually shown.
I thought about it some more, and there were three episodes. I checked my DVDs, and hunted up all the scenes I could find.
- Mudd's Women Season 1
Spock shows one of the crystals that was damaged in rescuing Mudd.
- The Alternative Factor Season1
Lazarus and crazy Lazarus both steal crystals from the Enterprise. You see the crystals several times, and also the dilithium chamber.
Here's the dilithium chamber:
Kirk throwing crazy Lazarus through his UFO into the corridor between the universes (crystals visible):
Lazarus (not the crazy one) with the crystals in the alternate universe:
Elaan of Troyius Season 3
This is the one everybody remembers, probably because Elaan was wearing them as a necklace - and she spent most of the episode wearing costumes that would make a stripper blush. I can't believe it took me nearly 40 years to recognize the ham-fisted play on words "Helen of Troy" and "Elaan of Troyius"
Burned out crystal in the dilithium chamber - note that it is now in a the engine room proper - and that the crystal looks like the ones in "The Alternative Factor."
Elaan with the necklace (no one knows that it is of dilithium yet):
Elaan has left the bridge and come back. Spock discovers her necklace is made of dilithium.
Spock and Scotty installing the "crude crystals" in the dilithium chamber. If you look, you'll see the burned out one lying there. It looks like the ones from "The Alternative Factor" but it is burned looking.
Elaan leaving the Enterprise. Notice she's wearing her necklace again. Kirk must have given it back.
It seems the props folks didn't have a real clear idea of what the dilithium crystals should be and what they should look like.
Besides the three episodes that were previously mentioned, I believe there is a brief scene in The Paradise Syndrome. I've only seen it once, as it seems to have been deleted from most syndication packages, but it's when Enterprise is closing in on the asteroid. Scotty does a quick visual inspection of the crystals where he reports to Spock, "Dilithium crystal circuit's failing, sir."