So, you'll be glad to hear that my cousin (he's the one who works in Hatton Garden, and he's a proper jeweller, who has been in the trade for 30 years) has gotten back to me. Anyway he is heavily invested in this ring story and was delighted to get anther photo!
He said (and I quote) that he'd now bet his kids lives that this ring is moissanite, i.e. lab grown silicon carbide). It isn't a diamond.
He says that the shine on it - 'the refractive index' is the giveaway, a diamond doesn't shine like a blag ring. He reckons that if they went to a good jeweller it is worth about 1.5K. max.
He also repeated that a real diamond this size would cost between £200k and £300k and couldn't be insured unless you agreed to security, basically. Celebs often have a fake one made just to get round that issue and keep the real one in a safe.
Anyway the short story is IT IS A BLAG RING.
Let me say this, what a sad life, Katie Jane. Blag ring beacuse you needed it to be big for Insta. Now too ashamed to show it as it is fake like your retouched photos.