LSD, a 'hit' of 'acid'
A small square of dried translucent substance containing a ‘hit’ of acid, that is, LSD, and resembling a ‘gel’ used in theatrical lighting. It dissolves in liquid, melts in your mouth.

A form of acid from the 70s. It came in colors (pun intended) such as amber and brown. Hence the name Amber Windowpane.