Hesalite Crystal - What is the Black Ring?

Hello,
I have a question about the Hesalite crystals, specifically about the black ring on them. I have a recent acquired 175.0032 Speedmaster Reduced - got it in really bad shape and have been slowly repairing it back to life.
Crystal cleaned up nicely and installed fine. What concerns me, as I don't know what its function is, is the black ring at the base of the crystal.
What is this? Is it a hard plastic gasket? For aesthetics? For water resistance? Something else?
I appreciate your insights!

A tension ring reinforce the edge of the crystal to make it fit tighter. Improve durability and water resistance.

Aha! Thank you. And thank you @350 XJR.

Of course!
So the project started with buying a broken 175.0032 off eBay for a little more then 1/2 the eBay going rate for one in good condition. When I say broken, I mean corroded movement, missing rotor, rust build up all over the interior of the case, missing chrono sweep hand, beat up dial, lume plots falling off, case filled with oiled (getting all over hands and dial), dinged and scratched case, and broken clasp.

From what I read, I think most people would have said good riddance to a Reduced speedie in that condition. Crazily, not me. I saw a terrific opportunity it’s to work on it.
So I fully disassembled, cleaned, de-rusted, determined what I could salvage, cleaned the dial and hands, relumed, polished and brushed the case & bracelet, repaired the clasp, cleaned the crystal, siliconed the gaskets, soldered on new dial feet, and replaced case back gasket. I still need to source a gen chrono sweep hand, replace the crystal tension ring, and I’m considering a new crown if water resistant testing doesn’t come back good.

I think it didn’t turn out half bad.

And here it is in the wild. As much as I started this thinking I would hate this watch, sell it off as quickly as I bought it (because it’s not a professional), I’ve come to like it. It’s a handsome watch and a nice size.

That's brilliant work! A pity to see such a neglected watch but the result is great. What did you use to remove the rust?

Impressive results! What’s your thinking regarding the relume. Was the original in so bad condition that it couldn’t be kept?

Thanks! I use Bergeon Rust Remover - https://www.esslinger.com/bergeon-30496-watchmakers-rust-remover/

Thanks! The lume was literally falling off due to the oil the previous person poured in. Actually some of the white lume plots came off too.
I feel like good lume plots are a defining characteristic of this dial. I was compelled to redo them. I don’t think I would have been satisfied with the restoration if I left the dial sans some of the hour plots and missing lume.

Thanks! It was a very enjoyable project.