Even if diamonds have the exact same strength and particle size, differences in their thermal stability will still exist, which is primarily caused by the different content of impurities (inclusions) within the diamond crystals.

Impurity content determines the initial weight-loss temperature: Experiments show that for multiple diamond single crystal samples with the same fracture load (strength) and particle size, their initial weight-loss temperatures vary significantly. This difference directly corresponds to the impurity content within the crystals: as the inclusion content within the crystal increases, the thermal stability of the diamond decreases accordingly.

Impurity content is the decisive factor for thermal stability: Whether comparing diamonds of the same strength or of different particle sizes, test results indicate that the thermal stability of a diamond depends heavily on its own impurity content. High impurity content is the main reason for poor thermal stability.

Internal stress destruction mechanism: As mentioned earlier, because the thermal expansion coefficients of inclusions (bubbles, catalyst metals, etc.) and the diamond matrix are different, shear stress is generated upon heating. The more impurities wrapped within the crystal, the greater the total shear stress generated when heated. This causes the diamond to reach its yield limit at lower temperatures, leading to plastic deformation and graphitization.

Therefore, even if two diamonds exhibit the same macroscopic mechanical strength at room temperature, as long as there is a difference in the quantity of impurities trapped inside them, their tolerance in high-temperature environments (thermal stability) will be different.