And now she has a proper invisibility item, except for the durability and the not being jewelry. Have any of her earlier enchantments worn off or degraded at all?

Not so far!

Sweet. She puts the invisibility rock in her backpack when she next goes to Dungeons and Dragons. What bizzare challenges will Xavier throw at them next?

It's apparently the underwater section of the adventure; they are given temporary use of magic items that let them maneuver and are sent to an underwater dungeon to retrieve an object.

That's seriously cool whether it has anything to do with the cultists plot or not. Also she should totally make a breathing-underwater artifact, someday when she's learned a lot more and also gotten an online reputation for reliable artifacts going. 

This dungeon has some neat traps. The one that starts up a vortex in the water to trap people is especially tricky and interesting.

The people who want the object turn out to have been infiltrated by cultists! They wind up fighting to hang on to the object and can now haul it to its probable rightful owner.

Well hang on, what is this thing? Because a lot of people seem to be after it, and if it's a bomb or the key to release a sealed horror or something then maybe even its probable rightful owner needs to end up disappointed.

It's a sealed box with who knows what inside. They can't get the box open even when Joseph insists on spending fifteen minutes trying different things and taking 20.

On the one hand, that's ominous. On the other hand, that's metagaming. And maaaaybe it's just really pretty jewels in a really secure jewelry box. What sort of person is this probable rightful owner presenting themself as?

They aren't, they're far away; they've determined her identity by reading the cult's correspondence.

Well, if she isn't actively pursuing it, then she probably isn't planning to use it to unseal any horrors right this minute. Margaret's character votes to bring it to her.

The party agrees, by and large, and they set out, and the session ends.

And Margaret catches up with Brenda on the way out again. "Can I show you something?"

"- sure? What is it?"

Margaret fishes the rock out of her backpack. "I have an invisibility rock! When I say the activation word I'll turn invisible without having to incant anything."

"- really?"

"Really really!" She murmurs. "Let me make sure I won't startle anyone--" is anybody else on this sidewalk if she looks around?

Yup, there's a family of lions going one way and a lone pegasus trotting the other way.

They all know magic is a thing, of course, but she doesn't want awkward questions from strangers. She steps into an alley between two houses to be marginally less obvious.

"Why are we hiding?" asks Brenda, slithering after her.

"I feel weird disappearing in front of people," she says, followed by "cacher".

Brenda gasps.

"Cesser." She's grinning when she appears again.

"Why is it in French?"

"If it was in English, I'd have to be careful not to say the word "hide" or "stop" in conversation while I was touching it or I'd go visible or invisible by accident. One less thing to worry about if it's in French."