Assign an emoji only when the emoji is a picture of the concept itself.

Good:    cow -> the cow emoji; apple -> the apple emoji; rain -> the rain cloud.
OK:      sad -> a crying face; stop -> a stop sign. Conventional signs are
         acceptable only when nothing depicts the concept more directly.
No:      cold -> a snowflake; money -> a dollar sign; idea -> a lightbulb.
         An association or a symbol is not a depiction.

Most words have no suitable emoji. "No emoji" is the correct, common answer -
prefer it over a stretch.