Raman microspectroscopy is an ideal method for the examination of marketable gemstones because of the lack of sample preparation involved and the nondestructive nature of Raman analysis

In recent years, the gemstone market has been flooded with stones of questionable origin. Frequently, even thorough analysis by a qualified jeweler cannot reveal unequivocally whether the gemstone is genuine or fake. In the worst case, sophisticated analytical methods struggle to differentiate modified diamonds, causing considerable concern to the international gemstone trade. Raman microspectroscopy is an ideal method for the examination of marketable gemstones because of the lack of sample preparation involved and the nondestructive nature of Raman analysis. The micro-Raman study of a stone also provides a unique record for identification purposes. This article discusses the variety of Raman spectra that can be obtained from different families of gemstones, comparing and contrasting spectra from genuine and artificial minerals. Color in gemstones is caused by light of specific wavelengths being absorbed inside the stone. Such absorption is a characteristic of the gemstone material, the molecular structure, and extraneous impurities or inclusions within the crystalline structure (1). The human eye perceives the different light wavelengths as colors. As white light enters a stone, the wavelengths are affected by the chemical and structural properties of the stone, some wavelengths being absorbed, others reflected. A combination of different wavelengths radiate from the stone, changing the white light that entered into emitted light of various colors (2). Materials formed in different configurations can have very few similarities. For example, both diamond and graphite are essentially pure carbon but the crystal structure of the carbon in diamond is completely different. For colored gemstones the quality of the color is worth at least 50% of the valuation of the gemstone (3). Color also comes from sources other than absorption of bands of light. Structural properties can cause various types of phenomena. The first is play of color, an effect caused by diffraction of light. In opal, which is made up of regularly stacked small spheres of silica, these layers of spheres act similar to a diffraction grating producing colors from the refracted light. The colors are determined by the sphere size and spacing. Iridescence is somewhat similar to a play of color in that the interference colors are created by light refracting from very thin layers of a gemstone material such as fir agate or ammonite. Orient is another interference phenomenon that occurs in pearls and is caused by the thin layer structure of the outer pearl and by undulations in nonspherical pearls. Asterism is caused by needle inclusions along crystallographic lines. This effect is seen in star rubies, sapphires, and some other stones. Chatoyancy is similar to asterism, but produces a single band of light caused by the needle inclusions creating the phenomena known as a catseye effect. Aventurescence is a sparkle effect caused by small plate-like inclusions and is common in aventurine quartz, sunstone feldspar, and Goldstone glass (4, 5).