I don't know how to lay things out properly, but luckily using way too much track saves the day.
The revari's wheel makes a copy of the input atom so that it can be sent to the output. I was originally going to make three copies to compare using purification, but then you can't detect gold, so I had to use sticks instead. There's a consistent anchor point for the stick with arm 13/14 to move it to the second station, where it moves in reverse - so it has an exact position if the current metal is equal to the last one. Then we just normalize the position and give arm 18 a lead if it wasn't equal. It's just straightforward conditionals from there and not really worth discussing.

The parts that process a single atom were made in-game, but without mods the instructions lag too much for me to even want to try to do the boundary special cases without omclone. If it wasn't for that, I'd do stuff like removing the unused track and replacing one of the wastechains with a disposal to improve the metric.
The main repeating loop to test one quicksilver on the input has an extra 2 cycle delay I added early on just in case I ever needed it. I didn't need it, but I didn't want to figure out how safe it would be to remove it.

Another approach I considered was making three copies of metals anyway since you can compare (including gold) compactly with quicksilver, but I felt like that would make the solution actually pretty slow. It'd save all the gold I spent on tracks though.