Here's why:

* High Melting Point: While diamonds have an incredibly high melting point (around 3550°C or 6422°F), this doesn't mean they're impervious to heat.

* Oxidation: Diamonds can oxidize (burn) at high temperatures, especially in the presence of oxygen. This process will damage the diamond and can cause it to lose its brilliance and clarity.

* Color Change: Heat can change the color of some diamonds. This is often done intentionally to enhance the diamond's color. However, uncontrolled heating can lead to unwanted color changes.

* Structural Changes: While diamonds are incredibly strong, prolonged exposure to extreme temperatures can cause structural changes. This can lead to weakening of the diamond and even fracturing.

Practical Considerations:

* Jewelry: While diamonds are used in jewelry, the heat from regular wear or even hot water is unlikely to cause significant damage.

* Industrial Applications: Diamonds are used in industrial applications where they are exposed to extreme heat. In these cases, special precautions are taken to protect the diamonds and prevent damage.

In short, diamonds are sensitive to heat, especially high temperatures. While they can withstand moderate heat, prolonged exposure to extreme temperatures can cause damage.