Having spent over 15+ years in the Jewellery industry, most of my knowledge comes from experience.
However, there are some amazing books, websites and social media accounts that I am in awe of and I highly recommend:

English Conversation About A Project
English Conversation About A Project


History of Famous Jewels and Collections
My No.1 favourite website for Royal Jewellery information. Brilliant resource for anyone interested in the history and provenance.
Royal Watcher
Very active on Instagram. Features many interesting articles on royalty and the Tiara section is one of my favourites for light reading.
The Court Jeweller
Fantastic website, Ella Kay has produced one of the most comprehensive sites on the web.
Order of Sartorial Splendour
This website is fantastic, especially the Jewel Central section.
Tiara Mania
A really detailed and in depth blog. There is an accompanying website but this isn’t updated as regularly as the Tumblr.
Queen’s Jewel Vault
From the author of the Order of Sartorial Splendour. Primarily focuses on Queen Elizabeth II. Very good for her vast collection of brooches.
Artemisia’s Royal Jewels
This website hasn’t been update since 2013, so I don’t know how much longer it will be in service. However, the information that is on there is still worth reading.
Royal Dish
More of a message board than a website but it does feature some very interesting articles.

Instagram Accounts

Spanish Royal Jewels

Monaco Royal Jewels

Greek Royal Jewels

H.E.R Jewels
Posts regularly about fine jewellery and royal tiaras.
Russian Treasure
This account focuses primarily on the Romanov’s and there astounding collection of jewellery.
A Tiara a Day
Truly does keep the doctor away.
Vincent Meylan
One of my favourite jewellery authors. His instagram is regularly updated with fascinating articles

There are some very detailed and accurate articles on Wikipedia relating to Jewellery. Some of my favourites include:
Diana’s Jewels
Elizabeth II Jewels
Joyas de Paysar
The French Crown Jewels