There are literally hundreds of differences in how they look and very often specific type is identified by a unique name. 

 In many cases this name comes from the specific area the rock was carved from. 

That is why we see name like: Laguna Lace Agate, Graveyard Point Plume Agate, Maury Mountain Moss Agate, Biggs Jasper, Regency Rose Plume Agate, etc. 

The name could also come from a description of what the rock looks like, for example: Black Feather Plume Agate, Tiger eye, Mahogany Obsidian, Blackskin Agate, etc.