Disclaimer: The specific numbers might be off, but the general point being made still stands. Carbon is a weird material.
In short: If it’s **just** heat being applied, then no, a diamond will not ‘melt.’
This is because diamonds are made of pure carbon, which, at standard surface air pressure (what you’re experiencing right now) does not have a liquid phase. Let’s look at three potential examples.
If you heat a diamond in the open air, it will eventually (at a temperature around 850 degrees Celsius or 1562 Fahrenheit, even lower if there’s more oxygen available) just burn. This happens as the carbon reacts with oxygen in the atmosphere and binds with it, burning the diamond until all you’re left with is carbon dioxide gas.
If you instead heat the diamond in a vacuum (while this happens gradually at any temperature, the temperature 1900 degrees Celsius/3452 degrees Fahrenheit is where this really speeds up) it becomes graphite. Diamonds are made of pure carbon that binds under immense pressure and heat, resulting in a sort of cage-like crystal structure (the shape the carbon atoms take when bound) made up of carbon atoms that are each bound to four other carbon atoms in a repeating series. Reheating these bonds in a vacuum without recreating the pressure just destabilizes them and converts them into the other pure form of carbon: graphite. Graphite’s crystal structure is made up of carbon atoms that are each bound to *three* other carbon atoms in a flat layer that stacks upon itself. The bonds between layers are very, very weak and layers will separate from simple friction, which is why graphite is so great for writing. If you keep heating the material even further (to around 3726.88 Celsius/6740.33, you’ll wind up with carbon gas as the graphite sublimates (the process of matter going directly from a solid to a gaseous state).
Now, if you instead set up an environment with a noble gas (or something else that won’t chemically react with the carbon and burn it) and (somehow) increase the pressure to an extreme degree (to roughly 98692.3 times the typical pressure you feel on Earth’s surface) and ramp the heat up to 4500 degrees Celsius/8132 degrees Fahrenheit, you will see a diamond melt. This is because carbon required both an immensely high temperature and pressure to enter its liquid state. I’m not sure as to whether molten carbon would be similar to molten glass, though.
Every substance on earth has what is called a “phase diagram”. This shows what temperatures and pressure will create certain phases of that matter (ie gas, liquid, solid). Diamond’s phase diagram shows that the only way for diamond to become liquid is for there to be .01 Giga pascal of pressure. That’s the 100x greater the air pressure at sea level. That also requires temperatures about 4500 Kelvin (about 7600°f).
So yes given enough heat AND pressure you could make liquid diamond.
However we can’t really replicate that on the surface of our planet. We can burn diamond though. The carbon bonds simply break down to do carbon gasses with oxygen.
Edit: even when it is a liquid you wouldn’t recognize it as a diamond. It would just be liquid carbon. All diamond is is a highly regular repeating structure of carbon to carbon bonds that can only be produced inside high pressure and temperature situations like inside out planets mantle.
The simplest answer is no, diamonds (pure carbon) under normal circumstances do not have a liquid phase. Apply enough heat, maybe some extra oxygen, and yes, you will see a diamond burn. Now, change the environment, put it in a vacuum, stick some gases in with it, etc, throw some heat in, and it’ll turn into graphite with different properties. A good place to research further woukd be the outer Gas Giants where it can actually rain diamonds in the thicker atmospheric areas, but what do they turn into as they approach the heat and pressure of the planets core?
A diamond is almost purely carbon.
Heating it sufficiently in an oxygen rich atmosphere it will simply burn. If you take away the oxygen and anything else carbon could react with you can create a liquid.
More precisely you need a temperature of at least 4600 K and a pressure of 11 MPa. For reference what you‘re experiencing rn is probably around 295 K and 100 KPa
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