Unidentified herbs were discontinued with the 2007 Herblore update. These herbs no longer exist and now only grimy and clean herbs exist. After the update, all unidentified herbs were changed to their respective grimy herb.
They were often called "unids" these were often bought in bulk for around 1-2k each in the hopes that the buyer would receive valuable herbs.
This was a simple way for players to trick others by identifying one herb then selling a pile of that herb (Unidentified versions) as a more valuable herb.