Diamonds in Neptune?
People have demonstrated high-pressure diamond formation using laser-driven shocks, much in line with the original idea. The diamond rain idea seems to be doing well (popular science article about it, with references). 

However, it is quite likely that the diamonds could have different properties because of formation mechanisms. There are a lot of unusual allotropes of carbon, and local conditions of formation (not to mention impurities) may cause the carbon to turn into these forms rather than the standard diamond lattice. 

Simulations show that beyond 1000 GPa diamond has a BC8 structure. However, this is a pressure unlikely to occur inside Neptune's atmosphere: it occurs deep in the mantle, so sinking diamonds may shift crystal form.