Sapphire, color

Several gemstone species occur in various colors, depending on the presence of impurities or irradiation-induced color centers. Examples are the beryl, comndum, and quart2 families. Quart2 has poor optical properties (RI = 1.55, DISP = 0.013), but becomes of gemological interest when it exhibits attractive colors. Any material can have its color modified by the addition of various impurities synthetic mby, sapphires, and spinel are produced commercially in over 100 colors (2). Synthetic cubic 2irconia has been made in essentially all colors of the spectmm (11), but only the colorless diamond imitation is produced commercially in any quantity. 

Some treatments are practiced so widely that untreated material is essentially unknown ia the jewelry trade. The heating of pale Fe-containing chalcedony to produce red-brown carnelian is one of these. Another example iavolves turquoise where the treated material is far superior ia color stabiUty. Such treatments have traditionally not been disclosed. Almost all blue sapphire on the market has been heat treated, but it is not possible to distinguish whether it was near-colorless comndum containing Fe and Ti before treatment, or whether it had already been blue and was only treated ia an attempt at marginal improvement. The irradiation of colorless topa2 to produce a blue color more iatense than any occurring naturally is, however, self-evident, and treatments used on diamond are always disclosed. 

Sapphyrins were discovered serendipitously by Woodward33 during the course of synthetic studies directed towards the total synthesis of vitamin B, 2. The sapphyrins were the first example of expanded porphyrins in the literature. Due to sapphyrin exhibiting a deep-blue color in the crystalline state and intense green in solution. Woodward coined the expression sapphyrin with reference to the deep-blue color of sapphire stones. 

The two extremes of ordering in solids are perfect crystals with complete regularity and amorphous solids that have little symmetry. Most solid materials are crystalline but contain defects. Crystalline defects can profoundly alter the properties of a solid material, often in ways that have usefial applications. Doped semiconductors, described in Section 10-, are solids into which impurity defects are introduced deliberately in order to modify electrical conductivity. Gemstones are crystals containing impurities that give them their color. Sapphires and rubies are imperfect crystals of colorless AI2 O3, red. 

As a contradistinction to the relatively simple case of AI2O3 Cr(III) where the color is due to a metal-centred electronic transition, we mention now on one hand the fact that the Cr(III) ion colors many transition-metal oxides brown (e.g. rutile Ti02 or the perovskite SrTi03 [15]), and on the other hand the fact that the color of blue sapphire (AI2O3 Fe, Ti [16]) is not simply due to a metal-centred transition. By way of illustration Fig. 1 shows the diffuse reflection spectrum of SrTiOj and SrTi03 Cr(III) [17], and Fig. 2 the absorption spectrum of Al203 Ti(III) and Al203 Ti(III), Fe(III) [18]. It has been shown that these colors are due to MMCT transitions and cannot simply be described by metal-centred transitions [19],... 

A colorless mineral known as corundum (composed of aluminum oxide) is colorless. A red variety of corundum known as ruby, a precious stone, owes its color to impurities of chromium within the crystal structure of corundum. Blue and violet varieties of corundum are classified as sapphires, the blue being the result of iron and titanium impurities, and the violet of vanadium impurities within the corundum crystal structure. Another colorless mineral is beryl (composed of beryllium aluminum silicate) but blue aquamarine, green emerald, and pink morganite, are precious varieties of beryl including different impurities aquamarine includes iron, emerald chromium and vanadium, and morganite manganese. 

Ruby and Sapphire. Ruby and sapphire are "sister stones" both are gemstone forms of the mineral corundum (composed of aluminum oxide). Pure corundum is colorless, but a variety of trace elements cause corundum to exhibit different colors. Ruby is red corundum, while sapphire is corundum in all colors except red. The red in rubies is caused by trace amounts of chromium the more intense the red color of a ruby, the more chromium it contains. The blue in sapphires is caused by titanium and/or iron impurities (Garland 2002 Hughes 1997). 

Barros, M. R. X., and Becker, P. C. 1993. Two-color synchronously mode-locked femtosecond Ti sapphire laser. Opt. Lett. 18 631-33. 

Leitenstorfer, A., Furst, C., and Lauhereau, A. 1995. Widely mnahle 2-color mode-locked Ti-sapphire laser with pulse jitter of less-than -2-fs. Opt. Lett. 20 916-18. 

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