Lithium niobate is a synthetic crystalline material with chemical formula LiNbO3, produced by the Czochralski crystal growth method. It is a recognised diamond simulant of historical significance and remains in limited use in lapidary applications, although in current trade it has been substantially displaced by the higher-performance simulants such as cubic zirconia and synthetic moissanite. Its principal commercial application is in the optoelectronics industry, in which it is used as a substrate for surface-acoustic-wave devices, optical modulators, frequency doublers and other photonic components.

Physical properties

Lithium niobate has a hardness of approximately 5.5 on the Mohs scale, which is below the typical hardness threshold for general jewellery use; the surface scratches readily and the polish degrades with normal wear. Refractive indices fall around 2.21 and 2.30 for the ordinary and extraordinary rays, with a high birefringence of approximately 0.09. Specific gravity is approximately 4.64, which is significantly higher than diamond at 3.52. The crystal is uniaxial negative and shows strong birefringence under crossed polars; the high birefringence is a diagnostic feature distinguishing the material from diamond on simple polariscope examination.

Visual character

Lithium niobate has a higher dispersion than diamond, at approximately 0.13 as compared with diamond's 0.044, which gives it strong fire when properly cut. The body colour can be slightly straw or yellowish in early production but later production developed colourless and various coloured material through doping. The high dispersion combined with the high refractive index gave the material a respectable appearance as a faceted simulant in the 1960s, when it was marketed under various trade names including Linobate.

The simulant market history

Lithium niobate occupied the diamond simulant niche briefly in the mid-twentieth century, before the development of cubic zirconia in 1976 and synthetic moissanite in 1996. The hardness was its most significant disadvantage relative to those later simulants; a lithium niobate stone would not retain its polish over typical wear cycles, which limited its acceptance as a sustainable jewellery material. Cubic zirconia, with its higher hardness and better wear resistance, came to dominate the diamond simulant market and lithium niobate retreated into specialty and historical-curiosity use.

Optoelectronic applications

Lithium niobate's lasting commercial importance is in the optoelectronics industry. Its piezoelectric and electro-optic properties make it suitable for the modulators used in fibre-optic communications systems, the surface-acoustic-wave filters used in mobile phones and other wireless devices, and the periodically poled lithium niobate (PPLN) crystals used in nonlinear optics for frequency conversion. The crystal growth and processing infrastructure that supports these applications is substantially larger than the gem-trade demand for the material, and lithium niobate boules are produced in industrial quantities for the photonics market.

Identification

Identification of lithium niobate is straightforward with standard gemmological equipment. The strong birefringence is visible on the polariscope. The refractive index is at the upper end of the gemmological refractometer's range. The specific gravity is well above diamond and well above most coloured stones. UV reaction is generally muted but doped material can show characteristic fluorescence. The combination of properties is unambiguous and the stone is rarely encountered in general retail goods today.