Youtube video of a car window being shattered by throwing a small chip of porcelain at it. It only takes a tiny nick to set the break in motion, and once it starts it's all over.
Casey

Here is a link that might be useful: Glass shatters from tiny impact

Normally glass is annealed - that's a process of slow cooling from working temperatures down to room temperature. For the glass we melt, a typical annealing cycle is something like the following:
2 hour soak at 960 (the annealing temperature)
8 hours from 960 to 840 (the strain point)
10 hours from 840 down to room temperature

Stress is created in the glass from uneven (or too quick) cooling. Proper annealing results in an object with minimal internal stress. When a properly annealed object breaks, the result is usually larger jagged pieces.

Tempered glass uses a very different process - in this case the glass is brought down in temperature very quickly, with the intention of creating an internally highly-stressed object. Contrary to what you'd think, this internal stress can make the object much stronger, however any scratch or chip can spread throughout the object in an 'explosive' fashion.

The result of a piece of tempered glass breaking is a billion little shards.

Note that as with most things glass, there is an 'art' to tempering - its not an exact science. Its entirely possible that one of the ovens at the factory was + or - some degrees or the glass formula that day was a little different and the result was your mug is not an strong as others made on the same production line.

I've linked the Wikipedia article on Rupert's drops, which are a great illustration of the potential explosiveness of tempered glass.

Here is a link that might be useful: Rupert's drops

your husband shot it with a bb gun, see the
hole in the wall above the intact mug?
LOL!
either that are they are haunted.
that's all I got!

I bet one of your mineral/rock/gemstones nicked the glass to trigger the break.
Casey

Tempered glass has very large internal stress created by quick cooling of the exterior and allowing the core to cool more slowly.

Flat sheets are created by heating and then cooling it rapidly.

It causes the fracturing into 'crumbs' (small pieces that are not large enough to fall and cut) when a scratch penetrates through temper layer and causes the stress to be released.

Once the crumbling starts it will not stop until nearly all the internal stress is relieved.
For windows the entire pane shatters, normally very quickly.
It can even make a loud sound similar to a basketball or soccer ball hitting the window.

Tempered glass was used in every car window except the windshield for many years.
It avoids having large sharp pieces of glass near people from an accident.
Windshields are 'safety' glass made by laminating plastic between layers of glass. They hold the piece together if the glass breaks, and the plastic prevent objects fr entering (or leaving) the passenger compartment.

I would bet Casey is correct.
A small nick (one you might not be able to easily see) probably got thing started.

Thanks, all. Certainly sounds like you're all right, about the small rocks causing it. I've learned a lesson about rock storage... Back to baggies or Tupperware.

Energy_rater, lol - I may (do!) have a couple holes in my siding from arrows, but nothing inside the house from anything. :-) That's 1 of 2 holes, from something before our time here, towel bar or such.

Have an explanation for the "shock" he felt, btw. I asked him "Hey, was that your left hand you used to pick up the glass piece?". He had a forehead-thumping moment & a chuckle at himself. Years ago, he cut his hand open - surgery reattached what nerves, tendons, muscle, etc, that they could, but he'll never have feeling in part of a few fingers. More to the point - occasionally any random thing can touch something "just right", & he never knows what different bizarre sensation he'll feel. We decided a sharp point must've touched a malfunctioning nerve, which felt like an electrical shock going up to his elbow.

So, all aspects of mystery are resolved. Thank you all for your patience & advice on such an off-topic question!

KS, you post and pose some of the most interesting things on this site! :)
I can sympathize with your husband on the nerve thing--got a cut at the base of my thumb some years ago from a cat food lid! Took three stitches, and from that time on the entire right side of my left thumb has no feeling in it. My cats now eat dry food or an occasional piece of ham or turkey, and I always use something to mash down the garbage in the bin! :)

CG, lol - & no comments from anyone about arrows into side of house... I expected a joke or 2 about tribal wars with the neighbors, or something! But then, who doesn't shoot an arrow or two into the side of their house?

FTR, I was teasing DH - about supernatural (ghostie) causes... Forgot he'd seen one in junior high. In his bedroom, for about 5 minutes - till she walked back to the other side of the room & gave him a clear path to the door. Which he took, at a run. (Lucky guy - I've never seen one!).

no comments from anyone about arrows into side of house...

let's hear it

Gotta watch out for Indian attacks in the West, you know--especially since somebody dropped a house on the Wicked Witch--so the crime rate is up! :) Aren't those flying monkeys a b*tch just after supper?

Ha - after supper is better than first thing in the morning, monkeys flying before breakfast? (Oops - never mind - those are my kids.).

And that "falling house" thing? That was just added to the documentary film, for drama. As you can see, I'm still here. :-D

As for the arrows... Actually, Ted Nugent lives next door, & REALLY hates loud music. That damn yankee really gets to storm-trooping at 1:00 am if my Neil Diamond cd volume gets cranked up. Arrow #2 was during Barry Manilow - so it wasn't just because he thought Neil Diamond was cheesy, he clearly just doesn't like loud music in general.

A few years back my wife's car was t-boned in a grocery parking lot. All was repaired. The car has a sun roof. A few days after we got the car back, as I was driving up the street there was a very loud boom. I thought some one had rear ended me. Turns out the sun roof glass had shattered and was hanging there in shards. Evidently an unknown result of the wreck.

A fellow employee had a Datsun with removable glass sections in the roof (AKA 'Targa top').

The removable sections are nice thick tempered glass, and even come with cloth bags to store them.

He bumped an edge trying to slide it into the cloth bag on the edge of the rear deck.
The 'explosion' scared him pretty well as the piece turned to glass 'crumbs' in his hands.

There is a LOT of stress built up in tempered glass (on purpose).