Diamonds are often assumed to be impervious to all forms of damage, including intense heat. While a diamond is the hardest natural substance, this property relates to its scratch resistance, not its behavior when exposed to fire. Diamonds will neither melt under common fire conditions nor are they indestructible. Instead, they are subject to a chemical reaction that causes them to burn away.
Why Diamonds Don’t Melt
Melting involves a material transitioning from a solid state to a liquid state, requiring energy to break the bonds holding the crystal structure together. Diamonds are composed of pure carbon atoms, bonded covalently to four neighbors in a rigid, three-dimensional lattice. This structure creates some of the strongest chemical bonds found in nature.
The energy needed to overcome these covalent bonds is immense, resulting in an estimated melting point of over 7,200°F (4,000°C) under high pressure. Achieving a liquid state requires temperatures far hotter than almost any fire encountered outside of specialized industrial settings. Long before this melting point is reached, the carbon atoms in the diamond undergo a different transformation when exposed to heat and air.
The Process of Oxidation
When exposed to high heat, a diamond undergoes oxidation, which is the scientific term for burning. Diamonds, made of carbon, react with oxygen in the air, transforming the solid crystal into a gas. This reaction is similar to how wood or coal burns, but it requires much higher temperatures due to the diamond’s tightly bonded structure.
The oxidation reaction begins when a diamond is heated to approximately 1,292°F to 1,562°F (700°C to 850°C) in ambient air. The carbon atoms combine with atmospheric oxygen (\(O_2\)) to form carbon dioxide (\(CO_2\)), a colorless gas. The chemical equation for this process is \(C \text{ (diamond)} + O_2 \to CO_2\).
The temperature required for combustion relates directly to the amount of oxygen present. In a laboratory setting with pure oxygen, the reaction can begin as low as 1,274°F (690°C). Since standard air is only about 21% oxygen, higher heat is needed to sustain the reaction. This difference explains why a diamond rapidly disappears in pure oxygen but typically only burns on the surface in normal air.
Real-World Fire Exposure
The effect of fire on a diamond depends on the intensity and duration of the heat source. A typical house fire may reach temperatures up to 2,000°F (1,100°C), which is hot enough to initiate oxidation on the diamond’s surface. However, the heat is often not sustained long enough to completely consume the stone.
Partial damage, such as surface etching or clouding, is more common in real-world scenarios. This damage results from the initial stages of oxidation, leaving a white or opaque appearance sometimes called “fire damage.” Jewelers’ torches can also reach the critical temperature for oxidation, requiring protective measures like boric acid coatings to shield the diamond during repairs.
A diamond scorched in a fire can often be restored by polishing away the damaged surface layer. This process removes the oxidized carbon, revealing the clear material beneath. While effective, restoration results in a slightly smaller and lighter stone.
Comparing Diamonds to Other Gemstones
The diamond’s unique reaction to fire sets it apart from most other popular gemstones, which are non-carbon-based mineral compounds. Gemstones like ruby and sapphire are varieties of corundum, or aluminum oxide (\(Al_2O_3\)). These stones are chemically stable and possess extremely high melting points, typically around 3,722°F (2,050°C).
Unlike diamonds, these oxide-based gemstones do not burn or oxidize in fire. Their primary risk from extreme heat is thermal shock, which occurs when they are heated or cooled too quickly. Rapid temperature changes can cause internal stress fractures, leading to cracking or cleaving. Other stones, such as emerald, may decompose at lower temperatures than the diamond’s combustion point, but they do not turn into gas.