(1) Inclusions
Alloy inclusions of various morphologies are present, appearing as rounded, rod-like, plate-like, or needle-like shapes; their arrangement correlates with internal growth zone boundaries. Inclusions may also be dispersed throughout the crystal as fine particles. These inclusions are opaque and exhibit a golden-yellow or black appearance with a metallic luster under reflected light.
Seed crystals and seed crystal phantom zones: The seed crystal phantom zone is a tetragonal single-pyramid growth zone formed by outward growth from a square seed crystal plate; its edges are defined by relatively bright, fine lines. This phantom zone persists regardless of whether the seed crystal plate is removed during processing, and the phenomenon is most clearly observed under dark-field illumination or by immersing the diamond in a liquid.
(2) Color
The vast majority of synthetic diamonds are yellow or brownish-yellow (most common) and exhibit hourglass-shaped color zoning, whereas natural diamonds range from colorless to pale yellow and other colors. However, high-quality CVD diamonds can also appear colorless or light brownish, making them nearly indistinguishable to the naked eye.
(3) Absorption Spectra
Synthetic diamonds lack the 415 nm absorption line that is most characteristic of the colorless-to-pale-yellow series of natural diamonds. Additionally, while the vast majority of natural diamonds are Type Ia, synthetic diamonds are primarily Type Ib, though nitrogen aggregates (pairs of nitrogen atoms) may be present in rare instances.
(4) Anomalous Birefringence
Under crossed polarizers, natural diamonds exhibit complex anomalous birefringence patterns-such as irregular bands, undulations, patches, and grid-like structures-due to the complexities of their growth and transport history. In contrast, synthetic diamonds show weaker anomalous birefringence, with some displaying intersecting bright bands in a cross-like pattern.
(5) Luminescence Characteristics
Ultraviolet (UV) fluorescence: Some synthetic diamonds are inert under long-wave UV light but exhibit moderate to strong yellow-green fluorescence under short-wave UV light, accompanied by distinct zoning-characteristics that differ from the fluorescence patterns of natural diamonds. Cathodoluminescence (under electron excitation):
Synthetic diamond: Color ranges from yellow to yellow-green; exhibits regular zoning (distinct differences between cubic and octahedral growth sectors).
Natural diamond: Predominantly blue; exhibits layered growth or complex growth patterns.
(6) Crystal morphology and surface characteristics
Synthetic diamonds typically occur as combinations of octahedral and cubic forms, and may also develop crystal faces such as rhombic dodecahedrons, tetrahexahedrons, or tristetrahedrons.
Surfaces may display dendritic growth lines, irregular hillocks, or knobby protrusions-features that differ from those found on natural diamonds.
