A distinctive feature of the natural origin of colorless, almost colorless, yellow, and yellow-brown diamonds is the fact that N3 centers, which are uncommon to synthetics, show up in absorption and luminescence spectra of 95% of natural diamonds. Type Ia diamonds containing these centers (so-called "cape series" diamonds) have a sharp absorption peak at a wavelength of 415 nm. Similar peaks at 423, 433, 452, 465, and 478 nm may also occur. The absorption spectra of natural type Ia diamonds sometimes contain weak peaks at 496 and 503 nm. Type Ib+IaA diamonds show no sharp absorption peaks. Natural diamonds colored as a result of treatment may have some additional sharp absorption peaks besides the above-mentioned ones: in type Ia diamonds these occur at 496, 503, and 595 nm, while in type Ib+IaA diamonds - at 503, 575, 595, 615, 625, and 637 nm. The absorbance of a diamond increases in the short-wavelength spectral region. The more intense the yellow or brown coloration of the diamond, the stronger this increase.
Absorption spectra of yellow, yellow-brown, and brown synthetic diamonds, measured at room temperature, usually have no sharp peaks. The key feature of these spectra is a shift of the absorption edge into the visible region. The shift is especially strong for heavily colored samples. However, this effect cannot serve as a reliable diagnostic feature. Therefore, it is recommended to measure the absorption spectra of diamonds at liquid nitrogen temperature when performing diagnostics of their origin. Under these conditions, the absorption spectra of synthetic diamonds (especially treated ones) contain a large number (up to 20 or 30) of narrow peaks: for type Ib+IaA diamonds the most important sharp peaks lie between 600 and 700 nm (namely, at 617, 627, 637, 647, 649, 658, 671, and 691 nm); type Ib or type IaA treated diamonds have sharp absorption peaks at 503, 527, 553, 575, 595, 617, 637, and 658 nm; absorption peaks at 437, 478, 481, 503, 511, 518, 527, 547, and 553 nm sometimes show up for type IaA treated diamonds.
There are no sharp peaks in the absorption spectra of colorless synthetics and blue natural (non-treated) and synthetic diamonds.