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.
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