Thanks for those links Jim and Maarten - a couple of sources there that I hadn't used before. 

I think you are correct Maarten - un écrin somptueux = a sumptuous setting

Hi G,

Have you tried here? :-

https://www.cnrtl.fr/definition/%C3%A9crin

A number of possibilities -- Boîte, coffret, jewellery box. Have a look at the link for all meanings then you can assess the context for your particular query.

Hope this helps.

Jim

Haven’t done the full exercise, but from the audio, I think it is referring to the figurative, environmental meaning - a (verdant) setting or background/backdrop.

https://dictionnaire.lerobert.com/definition/ecrin

https://www.wordreference.com/fren/écrin