Clipping off a bit breaks the enchantment.

Current working hypothesis: anything that makes it cease to be one object breaks the enchantment. She re-enchants it and continues down the list.

Unwinding doesn't disenchant the object till the rock comes off. Putting it back on doesn't reenchant it.

Re-enchant again and

* scratch the rock a bit

* crush a bit of the wire between pliers so it has little marks on it

Scratching has the first really interesting result - the enchantment glitches, turning her invisible and then failing, then turning just the arm on which she's wearing the ring invisible but fading back in at the shoulder; then it turns her invisible and seems to hold, but only a few minutes, before it flickers off again.

Seeing one arm go invisible but not the rest of her draws an "Ack!". Once it seems to have settled on "off" she takes the rock out again to break the enchantment. Does it still disenchant and re-enchant normally?

This works as expected.

Her notes now include "consider rings with small/no stones" and "re-enchantment warranty??". She moves on to crunching up the wire a little.

That works about like scratching the rock, though this time more of her is invisible when it flickers through a partial.

Disenchant, reenchant, all those invisibility diagrams she made a few weeks ago are really coming in handy now. Next test: normal wear and tear. She wears the ring while she does her homework the next evening, not being particularly careful with it but not deliberately whacking it on anything either. Assuming this doesn't break it, she idly fidgets with it on the table for a while, spinning it and flicking it back and forth and suchlike.

It doesn't break while worn or fidgeted with.

The next day is a Saturday; she wears the ring to the Avalon and is again only an ordinary amount of careless with it while she sits on a park bench working on a durability diagram and eats lunch at the Chinese place and so forth.

It persists in behaving itself when she checks.

She escalates to dropping it on the sidewalk, at some point when nobody's looking.

It bounces. It still works when she picks it up.

Brenda buys good materials. She goes back to working on the durability spell, with occasional breaks from stoichiometry to work on the incantation. When French ceases to be sufficient as a brain break, she walks around a bit. She has a general sense of the layout of the Avalon by now, but hasn't really seen most of the places in it aside from the library, the park, some restaurants, and Brenda's and Xavier's houses.

The Avalon has an arcade, a bookstore, a playground that looks like a poster for universal design, a community center currently in use for a lecture on Zen philosophy, a library, a professional-sized kitchen people can buy or subscribe for access to, a post office right near the entrance, stores which mostly have pretty limited selections plus ways to make special orders, a co-working space, a little park including a generic sports field in which some mad magical variant of polo is ongoing, a barber/groomer, a council building, a small black box theater advertising a run of "Cats" showing in the evenings and a few movies at various times during the day, a fortune-teller, and a one room schoolhouse.

. . . Margaret plays on the playground for a bit. Call it more wear-and-tear testing. She goes home before it's dinnertime, though, and by Sunday afternoon she has a draft of a durability spell ready. She's still following the "wait two days and check everything over" procedure, of course, and the incantation isn't quite done yet, but she'll be able to test it before game night rolls around. The incantation translates to "Make all components of this ring durable and strong against damage; let nothing break or alter them." 

Casting this spell does not kill her at all!

But does it make the wire of the ring harder to scratch up?

Yup, she can't get anywhere on that with the tools available.

She tries scratching the stone with her pliers, too.

Nope!

Excellent! Just for completeness, when her parents are in bed she'll take it down to the unfinished basement and whack it with a hammer, trying to break the stone. (Any warrantee she offers is not going to cover hammer-smashing, but she wants to know anyway.)

Her hammer bounces off and the ring goes flying into a pile of basement junk!