Essex geology is represented in Museums in and around Essex and in visitor centres at Essex Country Parks and at the Essex Field Club at Wat Tyler Visitor Centre at Pitsea.
Saffron Walden Museum has a geology gallery, with good reference material.
Braintree Museum has a few geological specimens and helpful staff.
Colchester Natural History Museum has geological displays and a microscope you can use.
Ipswich Museum, although over the border, has a very good reference collection relevant to Essex geology. See this before it disappears!
Chelmsford Museum at Oaklands Park Chelmsford used to have a geological reference room, but this has been closed. We have yet to discover what geological specimens and information will appear after the renewal program.
Hertfordshire Geological Society does very much the same thing for Hertfordshire as ERMS does for Essex. They also have an excellent new website.
This link brings you a great selection of gem information resources – a great way to explore more deeply into precious natural materials.
If you need good quality minerals and fossils try The Rock Gallery. Some Essex materials can be found here too.