So what kind of cool/useful things could we make out of diamond if diamond were suddenly very cheap?
This is the titular conceit of Neal Stephenson's novel the Diamond Age. In that fictional futuristic word almost anything can be manufactured in nanotechnological "material compilers". And according to the novel if you can do that, at scale it is cheaper, and easier to build transparent panes for windows out of diamond than glass because the chemical structure is simpler.
http://www.popsci.com/technology/article/2011-05/new-diamond...
If we assume a 1m³ vacuum container at sea level, that's displacing ~1200g of air. So that's the upper mass limit on the diamond container (or foam) for it to be neutrally buoyant.
Let's assume the density of diamond is 3.5 g/cm³, which means the volume of diamond "skin" we have to work with is something less than 342 cm³ for whatever gets built.
A sphere with a volume of 1m³ has a surface area of ~48,300 cm², which means we can't go thicker than 0.07mm. The most common thickness for mechanical pencil refills is 0.50mm.
That said, I'm sure there is some square-cube law business in favor of much smaller chunks, like a bubble-foam of smaller vacuum-cells.
Diamonds have lots of use in industry.
Answering to GP: diamonds happens to be really beautiful too, very shiny but not too shiny and not "not really shiny" like many lesser stones (like zirconium).
Many people like diamonds. Blame society, blame anything you want. But diamonds have fanclubs since thousands of years.
Diamond bearings?
IDK if the technique can create such a large crystal though.
But what will we do about our new existential crisis... If we no longer need to dig for diamonds, why would we dig at all?