Updated: August 15, 2026

Scientists study diamond and carbon under extreme pressure and temperature to understand unusual high-pressure phases and planetary interiors.

Diamond Melting Under Extreme Pressure Explained

Diamond melting under extreme pressure is a remarkable problem in high-pressure physics because carbon does not behave like an ordinary material when subjected to enormous pressures and temperatures. Under the right conditions, a diamond crystal can transition into a liquid carbon phase, and experiments have shown that the resulting fluid can have unusual electrical and density properties.

Scientists investigate this behavior using techniques including shock compression and laser-heated diamond-anvil cells. These experiments help researchers understand carbon's phase diagram and may provide clues about the interiors of planets such as Uranus and Neptune.

Can Diamond Actually Melt?

Yes. Diamond is a solid form of carbon, so at sufficiently high temperature and appropriate pressure it can enter a liquid state. The important point is that pressure changes the conditions under which carbon melts.

At ordinary atmospheric pressure, diamond is not simply heated until it behaves like a normal melting solid. Diamond is thermodynamically unstable relative to graphite at ambient pressure and tends to transform rather than undergoing ordinary melting under everyday conditions. Under extreme pressure, however, diamond can remain stable and eventually melt when temperatures become sufficiently high.

What Happens to Diamond Under Extreme Pressure?

Pressure changes the structure and thermodynamic stability of carbon. At pressures of several gigapascals and above, diamond becomes an important stable phase of carbon, while at much higher pressures researchers predict additional carbon structures and complex phase transitions.

The behavior becomes particularly unusual when pressure and temperature are increased together. Instead of simply becoming a conventional liquid, carbon can form a highly compressed fluid with electronic properties that differ dramatically from those of ordinary carbon-containing materials.

At What Temperature Does Diamond Melt?

There is no single melting temperature for diamond because the melting point depends strongly on pressure. Researchers therefore describe diamond's behavior using a melting curve rather than one fixed number.

Experiments and theoretical calculations have produced different versions of this curve, especially in the lower-pressure region. One experimental study using laser-flash heating reported evidence of diamond melting between approximately 13 and 50 gigapascals and concluded that diamond melts below the graphite-diamond-liquid triple-point temperature in the pressure range it investigated.

At much higher pressures, shock-compression experiments have examined carbon between approximately 0.60 and 1.05 terapascals. Those measurements indicate that diamond can melt into a denser metallic fluid and that the melting curve has a negative slope in that extreme-pressure range.

Why Does Pressure Matter So Much?

The melting temperature of a material is determined by the balance between its solid and liquid phases. Pressure changes the relative volumes and energies of those phases, so increasing pressure does not necessarily make the melting temperature rise in a simple, predictable way.

Diamond is a particularly interesting example because the relationship between pressure and melting changes at very high pressures. Earlier models often predicted a steadily increasing melting temperature, whereas later research identified a maximum in the melting curve and more complicated behavior at megabar pressures.

Diamond's Extreme-Pressure Melting Curve

The carbon phase diagram contains several regions corresponding to different physical forms of carbon. Diamond, graphite, other predicted high-pressure structures and liquid carbon occupy different parts of this diagram.

A notable reference point is the graphite-diamond-liquid triple point, estimated in one experimental study at roughly 13 GPa and 4,000 K. Researchers subsequently investigated melting behavior at pressures extending to around 50 GPa, illustrating how complicated the boundary between solid and liquid carbon can be.

At still higher pressures, calculations predict additional carbon phases such as BC8, and theoretical work places a possible diamond/BC8/liquid triple point near 850 GPa and 7,400 K. These values illustrate just how far laboratory conditions can extend beyond those encountered in ordinary materials science.

Does Molten Diamond Become Metal?

In the high-pressure regime, molten carbon can exhibit metallic electrical behavior. First-principles calculations predict that hot, compressed diamond is initially a semiconductor and undergoes metallization upon melting. The resulting liquid retains some covalent character while behaving as a conducting fluid.

Shock-compression experiments at pressures around 0.6 to 1.05 terapascals similarly found evidence that melting produces a denser metallic fluid. This is one of the most unusual aspects of diamond melting: the liquid is not simply an extremely hot version of familiar carbon.

How Do Scientists Melt Diamond in the Laboratory?

Researchers cannot simply place a large diamond into a conventional furnace and reproduce these conditions. Instead, they use specialized high-pressure techniques.

Diamond-Anvil Cells

A diamond-anvil cell squeezes a microscopic sample between two opposing diamond anvils. Extremely high pressures can then be generated inside a tiny sample chamber. Laser heating can raise the sample's temperature thousands of degrees above room temperature.

Research published on laser-flash heating has used diamond cells to investigate melting between roughly 13 and 50 GPa, combining the high-pressure apparatus with spectroscopic and electron-microscopy analysis of recovered material.

Shock Compression

Another approach is shock compression. A powerful shock wave rapidly compresses the carbon sample while simultaneously raising its temperature. This technique has allowed researchers to reach pressures far beyond those normally achievable with static laboratory compression.

Shock experiments have provided experimental information on diamond melting at pressures approaching a million atmospheres and beyond, including the 0.6–1.1 TPa range reported in major high-pressure studies.

Why Are Scientists Interested in Melting Diamond?

The research has implications far beyond diamond itself. Carbon is an important element in planetary science, and extreme-pressure carbon physics can help researchers model the interiors of giant planets and carbon-rich worlds.

Earlier high-pressure studies highlighted the importance of carbon's phase boundaries for modeling planets such as Uranus and Neptune, as well as white dwarfs and possible carbon-rich exoplanets.

The results can also improve scientific models used for extreme-condition physics, including calculations of equations of state and the behavior of materials under shock compression.

Does Pressure Make Diamond Harder to Melt?

Not necessarily. This is where diamond differs from the intuitive idea that greater pressure simply makes every solid harder to melt.

Pressure changes the relative stability of different carbon phases, and the melting curve can rise, reach a maximum and behave differently at ultrahigh pressures. Research therefore treats diamond melting as a pressure-dependent thermodynamic problem rather than assigning diamond one universal melting temperature.

Is Diamond Melting Fully Understood?

No. Carbon's phase diagram at extreme pressure and temperature remains an active research area. Experimental measurements are difficult because the required conditions can reach hundreds of gigapascals or even terapascals and thousands to tens of thousands of kelvin.

Researchers have historically obtained differing experimental and theoretical results, particularly concerning the precise position and slope of the diamond melting boundary. The continued use of improved laser-heating, shock-compression and computational methods reflects the complexity of the problem.

What Happens at the Highest Pressures?

At pressures approaching and exceeding a terapascals, carbon can enter regimes that are far removed from ordinary laboratory materials. Experiments have found that the diamond melting transition can produce a dense metallic fluid, while at still higher shock pressures the carbon fluid can undergo additional structural and electronic changes.

One study reported evidence that the complex fluid formed from diamond at high pressure eventually dissociates as pressure rises into approximately the 1.1–2.5 TPa range and temperature exceeds roughly 50,000 K.

Bottom Line

Diamond can melt under extreme pressure and temperature. However, its melting behavior is much more complicated than the familiar melting of ice or metal. Pressure changes carbon's phase stability, and at very high pressures the melting curve can behave unexpectedly.

Laboratory experiments have observed or inferred melting across several extreme-pressure regimes, including tens of gigapascals and pressures approaching a terapascals. At the highest pressures studied, molten carbon can become a dense, electrically conducting metallic fluid.

The science remains an active area of research because understanding carbon under these conditions can reveal fundamental properties of matter and improve models of some of the most extreme environments in planetary science.