You can melt glass and rock, so can you melt precious gems?

At atmospheric pressure rubies melt at 1 889 K, while diamonds will not melt. They will instead burn at ~973 K, and in vacuum they will change to the other solid allotrope of carbon, graphite, at 1 973 K. To melt a diamond, you need a pressure of 10.8 MPa (107 atm) and a temperature of ~4 600 K. read more

If you can melt a gem or not depends on the melting point. Soft glass has a melting point around 500 °C, so it relatively easy to melt. Ruby is chemically aluminium oxide, Al2O3 (with chromium inpurities, to give a red color) has a melting point of around 2000 °C, quite difficult to melt! Diamond is one of the different forms of pure carbon. read more

If you manage to achieve the conditions (temperature, pressure) to indeed melt it - whether that is doable depends on the type of gem -, it will become liquid. When it cools down and solidifies again, it will in all probability not be a gem any more, but an amorphous, glassy substance. read more

You can melt anything if you raise the temperature high enough, but don't expect to be able to 'cast' gem-stone. Those gems are single crystals, formed over a long time under specificvery high pressure. What solidified from your 'melt' would not be what you put in. read more