Facet Key visual specifying icons

I can't seem to specify icons for my facte key data.

I have tried using fa fa-building for example

I notice the docs on the site have changed https://fontawesome.com/how-to-use/on-the-web/referencing-icons/basic-use

This suggests I use fas instead of fa

I have tried the following

fas fa-building

fas fas-building

fa fa-building

all without success

I notice that fa fa-male still works as does fa fa-globe

is there any useful guidance on the list of available icon types/classes?