Suggest you contact the manufacturer for ideas.

Looks like paint....will it scrape off.....gently of course.

"Neither kid has any idea what it could be - or at least that’s their story and they are sticking to it."

lol. Smart kids; I've seen this before. Kids plugged a battery charger into the backsplash. It leaked and etched the estone. They put a large candle over the etch, blamed the candle, Dad bought the story, and called me back. DuPont flew a tech out twice and wrote a repair bulletin from that repair.

It's an etch and looks like a goodun'. A skilled tech with a diamond pad protocol and a Quartz Renew kit and the experience to use same may get you out of this. Don't be surprised if he makes you acknowledge the top is ruined in writing, insists upon cash payment in full before beginning, and offers no satisfaction guarantee. You can thank the pervasive online reviews for those policies.

The etch looks like a bottle bottom to me. Look around the house for a matching fit please, because the kids were probably not savvy enough to throw it out. Motorcycle chain oil remover, toilet bowl cleaner, something with hydrochloric acid maybe.

Was hoping you’d see this @Joseph Corlett. Toilet bowl cleaner would imply my kids willingly clean 😆

Will have to hunt good ol’ Kalamazoo for the aforementioned skilled tech. Wish me luck.

Are there questions I should ask a tech to assess their skills and knowledge?

If you're feeling adventuresome, clean it with acetone, over fill it with thin superglue, and spritz the superglue with the accelerator that you bought online or at the hobby store. Scrape the overflow away with a porcelain blade, available online, which won't make the metal marks a regular razor blade will. Hit it with a little polish to remove some of the scrape marks and try to match the factory gloss. That's the short version of the DuPont repair bulletin.

Don't worry, you're not going to make it any worse.

"Are there questions I should ask a tech to assess their skills and knowledge?"

The person you want will have a portfolio of pictures and satisfied customers that he's worked very hard to get and will love to show off and have you call, respectively. Confidence, and maybe even a little cockiness, is a good sign. You want to think "Well, either this guy is a good as he says, or he's the best bu!!$#!tter I've ever met." All or nuthin'.

Remember, you aren't the only one picking. Technical repair skills aren't nearly as important as the ability to assess the fussiness and sophistication level of your customers in running a successful repair business. While you're looking at my pictures, I'm looking to see if your floors are cleaner than the plates in my cupboard. If they are, your repair is probably too far gone for me.

Sure, I'd like the opportunity to earn a thousand dollars in several hours (I'm still saving you thousands over replacement), but you're actually asking me to bet my 4.65 customer satisfaction rating (out of a possible 5), that I can pull it off. That means lowering my risk as much as possible and that's done when I assess and pick you. If I miss and you're unhappy, I'm probably giving you your cash back. Combined with the before and after pictures I took and your written admission that the top was ruined before I started, that's kind of a slam-dunk response to your crappy online review. Use lots of caps and exclamation points please.

www.stonefabricatorsalliance.com has a geographical map of members; I'd try there.