You are correct, Methuselah Flint.
Well done! The game is yours.
Answer ~ the Food Tray in The Gamesters of Triskelion
Clue 1: The object is used by one of our heroes to try to help a native understand a fundamental concept.
Captain Kirk tries to explain to Shahna, a Thrall on Triskelion, what beautiful means by using the food tray as a mirror so she can see her reflection. Beauty is often referred to as one of the three fundamental concepts of human understanding, along with truth and goodness.
KIRK: It means you’re a very beautiful woman.
SHAHNA: What is beautiful?
KIRK: You mean nobody's ever told you that before? well, er, beautiful is, Beautiful. (he holds up the tray so she can see her reflection) That's beautiful.
Clue 2: This is not the primary purpose of the object, however.
Kirk uses the food tray as an impromptu mirror.
Clue 3: This object’s appearance is metallic but is not high tech at all.
It is a common ordinary metal food tray. The word appearance was included in the clue because of the unconventional use of the food tray.