De Beers relies on the fact that ordinary people don't understand diamonds aren't very valuable. After all, the diamonds they see sold by De Beers are expensive, so...?
De Beers plucks a $50 diamond from the mines, charges $5000 for it in jewellery and says that's symbollic of True Love, or whatever nonsense people believe.
But the people making diamond cutting saws do not care about True Love, so they are already buying whatever is cheapest, and they won't pay more than $10 for $10 of diamonds. To them that $50 diamond is worth... $50.
De Beers new plan is to make $50 synthetic diamonds, brand them, get across to consumers how these aren't "real" diamonds (hurting synthetic producers), that they're more "I wanna get into your knickers" than "True Love" and charge say $500 for them to emphasise that they're worse than its $5000 mined diamonds.
That makes no difference to the diamond cutting saw people, they still just want diamonds for their purely mechanical properties, and they'll continue to pay $10 for $10 of little diamonds.
De Beers hopes that this drives the synthetic diamond competition out of their market, and allows it to go back to charging $5000 for a $50 diamond in jewellery. The industrial market operates on a rational basis, there's no profit for De Beers there, it's all about the silly people paying $5000 to signal "True Love".
About flammability... Technically it is flammable. If you heat it enough, it will react with oxygen. Steel will melt way before that point, I would be it would burn before that point too.
Considering this, I don't think this is going to bring about the Diamond Age unfortunately.
Diamonds, despite being very hard, are also very brittle. They'll likely crack much more easily than Gorilla Glass (another hard material that is hard to scratch).