The coloration of colored artificial diamonds is fundamentally different from that of natural colored diamonds. Its core lies in the precise design and regulation of defects (including impurity elements and lattice vacancies) within the crystal lattice.

In terms of physics, pure and flawless diamond has a wide bandgap energy of 5.5 eV. Visible light (with energy less than 5.5 eV) cannot trigger bandgap transitions, allowing light to pass through completely, which makes the crystal colorless. When specific impurity atoms are actively introduced into the crystal lattice or controllable lattice defects are created, new electronic energy levels are introduced into the diamond's bandgap. These levels selectively absorb visible light of specific wavelengths, causing the diamond to present the complementary color of the absorbed light.

Based on the causes of coloration, the coloring mechanisms of colored artificial diamonds can be classified into three main categories:

1. Coloration by Impurity Elements
When atoms of other elements substitute carbon atoms in the diamond lattice, new electronic energy levels are introduced. This is the most fundamental coloring method:
Nitrogen (N) Impurities (Yellow Series): Nitrogen is the primary impurity in diamonds. When isolated nitrogen atoms (single nitrogen) substitute carbon atoms, they introduce a donor level (approximately 2.2 eV from the conduction band), selectively absorbing blue-violet light and rendering the diamond yellow. When the aggregation state of nitrogen atoms changes, the color changes accordingly: two adjacent nitrogen atoms forming the A-center lighten the yellow color; while larger aggregates forming the N3-center mainly absorb blue light, exhibiting a more vivid yellow; heavy nitrogen doping leads to deep yellow or even brown.
Boron (B) Impurities (Blue Series): When boron atoms substitute carbon atoms, they introduce an acceptor level above the valence band (approximately 0.4 eV), absorbing yellow-red light from the infrared to 500 nm range, which makes the diamond blue. The depth of the blue color is positively correlated with the boron doping concentration—light blue at low concentrations, deep blue at high concentrations, and blue-black at extremely heavy doping (>10²¹ cm⁻³).
Transition Metals and Co-doping: In High-Temperature High-Pressure (HPHT) synthesis, transition metals from catalysts (such as nickel Ni, cobalt Co) entering the lattice or remaining as metallic inclusions can produce yellow-green or brownish-green tones; additionally, boron-nitrogen co-doping can generate green or gray colors.

2. Coloration by Lattice Defects (Color Centers)
By using high-energy particle irradiation to knock carbon atoms into interstitial positions, creating vacancies, and then applying heat treatment (annealing) to migrate and combine these vacancies with impurity atoms, a series of color centers with specific absorption characteristics can be formed:
Nitrogen-Vacancy (NV) Color Centers (Pink, Purple, Orange-Yellow Series): Formed by a substitutional nitrogen atom adjacent to a vacancy. Among them, the NV⁰ center (zero-phonon line at 575 nm) absorbs blue-green light, making the diamond orange-yellow; the NV⁻ center (zero-phonon line at 637 nm) strongly absorbs yellow-green light and emits red fluorescence, which is key to producing pink, purple, and purplish-red diamonds.
GR1 Color Centers (Green Series): Represented by neutral, isolated carbon vacancy defects (zero-phonon line at 741 nm). It strongly absorbs red light and some green light, making the transmitted light predominantly blue-green, which is the primary mechanism for producing green diamonds.
SiV Color Centers (Blue-Green, Green Series): Consisting of a silicon atom and a double vacancy, it strongly absorbs yellow light at 738 nm, giving the material a vibrant blue-green or green appearance.
H3/H4 Color Centers (Yellow Series): When vacancies produced by irradiation are captured by aggregates of two or four nitrogen atoms, they form H3 (503.2 nm) or H4 (497.5 nm) centers, causing the diamond to appear yellow.

3. Coloration by Inclusions and Micro-defect Clusters
Black or Dark Gray: These colors are typically not caused by a single color center, but rather by high-density graphite inclusions, metallic inclusions, or massive micro-defect clusters within the crystal, which almost completely absorb light entering the crystal, resulting in a black or dark gray appearance.