@BattleDwarfGimli@jb There is a story dating back to Rome; I can't recall which Emperor (Tiberius, maybe) visited a man who created unbreakable glass. Long story short, he had him killed and his workshop and notes burned.
Unbreakable glass would be more valuable than gold, and would destabilize the currency. Gold's value is in being uncommon, easily workable, and resistant to corrosion. Unbreakable glass would be better than gold: common, workable, resistant.
To the People it would be better. Not the State. It would be a disaster. So he was killed. (It would also be a disaster for the People, but the damage would be done before they figured it out).
Tiberius really was that farseeing. And the technological leap was lost for nearly two thousand years.
@bloggerski@BattleDwarfGimli@jb At the time, a "cash economy" only existed in terms of what the State was doing. Most trade was local and conducted by means of barter. We have a hard time grasping this as we have put a pricetag on everything, but "the dole" was a handful of grain. Farmers might make no payments to farmhands at all, as everyone working needed food at mealtimes, which was coming directly from the farm. No paychecks.
The government payments to the legionairies was different, and comprised the bulk of the cash economy. They were both the army and the "civil engineers" as the soldiery built everything; aqueducts, roads, walls, everything was built by the legionairies who spent most of their time building and training and only occasionally had to fight.
They needed money to buy what they needed to get by, so their compensation was with coin (and salt).
Everything has an inherent, natural value, except money. If the money is accepted, it has value. If not, it's worthless.
Gold has some natural decorative value in that it's shiny, but its true value comes from its scarcity and rustlessness.
Unbreakable glass cast as coins would be better. Lighter, more durable, easier to cast, nicer to look at, readily available...
If the secret was guarded, the test for counterfeits would be merely trying to break it.
And that's just considering replacing the currency. What is the difference in trade value between old-thing which will wear down or break eventually, and new-thing made of newglass? Will the ironmongers be put out of business? The wood carvers? How many things will be replaced by Duraglass?
We saw this in the 20th Century when everything was replaced by plastic. Entire industries simply disappeared.
The Emperor had a moment of great clarity: keep things as-is and stable, or inject civilization-spanning change that destabilizes the world.
Rome prided herself on stability, so the choice was made.