Amber was a substance which sometimes contained the bodies of insects.
Polly Wright compared the TARDIS being in a vast emptiness to flies in amber. (PROSE: When Starlight Grows Cold)
Meglos compared the Fourth Doctor and Romana II, whom he had placed in a chronic hysteresis, to flies trapped in amber. (TV: Meglos)
The Eighth Doctor compared the people stuck in Edward Grove's time loop to flies caught in amber. (AUDIO: The Chimes of Midnight)
The Second Doctor reflected that Flinthair's body would one day be found trapped in ice like a fly in amber. (PROSE: Loop the Loup)