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Diamond, with its unrivaled hardness and brilliant luster, is known as the "king of gemstones"

February 29,2024

However, did you know that despite their names, they all have different crystal structures and impurity contents, and these differences make each diamond unique? Today, we will delve into the classification of diamond crystals and the relationship between impurities and their properties.

First, let's understand the classification of diamonds. According to its crystal structure and impurity content, diamond can be divided into four main types: type Ia, type Ib, type IIa and type IIb. These classifications are mainly based on the presence and content of nitrogen impurities in diamonds.

Type Ia diamond


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Type Ia diamond is the most common type of natural diamond, the crystal contains nitrogen impurities of more than 0.1% volume fraction, does not have paramagnetic characteristics, and the crystal is yellow. This type of diamond accounts for more than 98% of natural diamond production. It has a high nitrogen content and a specific crystal structure. This type of diamond has excellent mechanical properties and high electrical insulation, but poor thermal conductivity and relatively narrow light transmission bands. These properties are mainly due to the presence of nitrogen impurities in a non-paramagnetic manner in Type Ia diamonds, resulting in specific physical properties. Most natural diamonds fall into this category.

Type Ib diamond

In contrast, type Ib diamond has a comparable nitrogen content to type Ia, except that the nitrogen it contains is in the form of independent individual nitrogen and is present in a paramagnetic manner. Its thermal conductivity is much better than that of type Ia, and this type of diamond exhibits a more pronounced yellow color. It is also inferior in mechanical strength. Among synthetic diamonds,  type Ib occupies a significant proportion. It is extremely rare in natural diamonds. This is because in natural diamonds, the nitrogen as an impurity tends to accumulate over time due to the tendency of the crystal's minimum internal energy, and it is transformed from type I b to type Ia.

Type IIa diamond

Type IIa diamond has an extremely low nitrogen content, which is the main difference between it and other types. The interior of the crystal is colorless and transparent, and due to the reduced nitrogen content, type IIa diamond exhibits extremely high thermal conductivity and the widest transmission band. The thermal conductivity is 5 times that of copper at room temperature. However, this type of diamond is also relatively rare in nature, accounting for about 2% of natural diamonds. Today, scientists have succeeded in artificially producing this low-nitrogen, high-thermally conductive diamond.

Due to its high thermal conductivity, it is widely used in high-precision fields such as solid-state microwavers, lasers, high-power body tubes, variable reactance diodes, and heat sinks for semiconductor switching devices.

Type IIb diamond

Finally, there is type IIb diamond, which is characterized by a much lower nitrogen content than type Ia, but contains boron elements, which are very colorless and sky blue. This type of diamond has the properties of a P-type semiconductor, and this type of diamond is rare in nature. In type IIb diamond, the role of boron is mainly to replace carbon atoms and form boron-carbon covalent bonds, thereby changing the electronic structure and chemical properties of diamond. This boron-containing diamond has some unique physical and chemical properties, such as resistance to oxidation and heat. Experiments show that the initial oxidation temperature of boron-containing diamond is 150°C~250°C higher than that of ordinary diamond. At present, semiconductor diamond can be artificially produced by doping.

Type IIb diamond has high hardness and wear resistance characteristics due to its special crystal structure, so it can be applied to various cutting tools, drill bits and grinding wheels and other tool materials, as well as high-precision machining materials, to improve processing efficiency and precision. In addition, type IIb diamond has the properties of semiconductors, so it is also commonly used in the manufacture of electronic components such as rectifiers and transistors. Due to its high optical transmittance and low thermal conductivity, type IIb diamond can also be used in the manufacture of optical devices and lasers.

From the above analysis, we can see that there is a close relationship between the classification of diamond crystals and impurities. Different types of impurities and contents affect the physical and chemical properties of diamonds, giving them unique properties and values. This knowledge not only helps us better understand this amazing king of gemstones, but also provides an important theoretical basis for future research and applications. It will allow us to further explore its application potential in the high-tech field.