This link popped up on Pinterest - I know nothing about the family other than they are an old Scottish noble family and the current Earl is Lord Constable of Scotland.

His son and heir married in 2017 and the photographers at the wedding have a website which display lots of pictures of the bride and what I assume is an Erroll family tiara.

Its a large and beautiful tiara.

https://en.wikipedia.org/wiki/Merlin_Hay,_24th_Earl_of_Erroll

https://davidbostockphotography.co.uk/cotswolds-wedding-oxfordshire-wedding-photographer-clementine-harry/