Sapphire is mainly colorless, blue, green, yellow, orange, brown, pink, purple, gray, black and other colors. According to the color, corundum gems are divided into ruby and sapphire in the international jewelry industry. Traditionally, medium to deep red corundum gems are collectively referred to as rubies, and corundum gems of all colors except rubies are collectively referred to as sapphires. Light red corundum sapphire is classified as sapphire, so the names of light red sapphire and pink sapphire appear. In practical application, it is difficult to accurately distinguish the boundaries between pink sapphires and rubies. At the annual meeting of the International colored gems Association (ICA) held in Bangkok from 65438 to 0989, a new principle was put forward for the boundary between ruby and sapphire, that is, all corundum gems with red color were classified as ruby and corundum gems with other colors were classified as sapphire.
Sapphire is very rich in color, including almost all colors of orange, yellow, green, cyan, blue and purple in the visible spectrum (Figure 5-7). Corundum is a mineral of other colors, and pure sapphire is colorless, but most sapphires contain a certain amount of trace elements. These trace elements directly affect the color of sapphire. The color of blue sapphire is mainly because it contains trace amounts of iron and titanium. The coexistence of iron and chromium in sapphire can make sapphire appear golden or orange. If only chromium exists, pink sapphire and red ruby will be formed. Different combinations of trace elements create the extraordinary color of sapphire, which is commonly called colored sapphire. See Table 5-2 for the corresponding relationship between different trace elements and colors in corundum.
Figure 5-7 Colorful Sapphire in Various Colors
Table 5-2 Corresponding Table of Different Chromogenic Elements and Colors in Sapphire
The colors of sapphires from different places of origin also have certain rules to follow. The following will describe sapphires produced in different places according to the color series.
1. Blue Series
Blue series is the most widely distributed color series among sapphires (Figure 5-8). Almost all corundum producing areas in the world have blue sapphires. Although the mining areas such as Wardak in Afghanistan and Wenza in Tanzania mainly produce rubies, they also produce a small amount of blue sapphires. Sapphire is the most widely distributed color variety in the whole corundum family.
Figure 5-8 Blue Sapphire
2. Yellow series
Sri Lankan yellow sapphire has a light yellow to medium yellow hue and does not have any brown hue. Some darker shades can produce deep yellow, gold and orange, which are very rare in natural yellow sapphires after heat treatment. The very rare pink-orange Padma sapphire is mainly produced in Sri Lanka and Vietnam. Brown-orange sapphires similar to Padma sapphires can occasionally appear in Umba mining area in Tanzania. High quality yellow sapphires made in Thailand can reach golden yellow to orange. The yellow sapphire in Queensland, Australia is green. Sri Lanka, Thailand and Australia are the main producing areas of deep yellow sapphires (Figure 5-9).
Figure 5-9 Yellow Sapphire
3. Green series
Although the best quality green sapphires come from Sri Lanka, they are very rare. Sri Lankan green sapphires are lighter in color than those from Thailand, Australia and Shandong, China. The color of green sapphires in Thailand, Australia and China tends to be blue-green or yellow-green, and the gray scale is heavier. Green sapphires with carat quality exceeding 10ct and good color and clarity are very scarce, but the market demand is not great (Figure 5- 10).
Figure 5- 10 Green Sapphire
4. Purple series
This series mainly includes violet and purple sapphires, mainly produced in Gu Mo, Myanmar, Sri Lanka and Viet Nam. The price of purple sapphire is even comparable to that of ruby.
5.BGY sapphire
Sapphire in eastern Australia, Berlin in Thailand-Cambodia, Changle in Shandong, China (Figure 5- 1 1) and Anziranana in northern Madagascar all belong to basalt magmatic rocks. Sapphire produced in these mining areas is mainly blue/blue-purple-blue-green/yellow-green-yellow, so it is called BGY sapphire, that is, blue-green-yellow sapphire.
Figure 5- 1 1 Shandong Changle BGY sapphire
(2) The reason why sapphire is colored.
The color of blue sapphire is mainly caused by charge transfer between valence states of heteronuclear atoms. In blue sapphire, Fe2+ and Ti4+ are located in adjacent octahedrons connected by faces. The distance between Fe and Ti ions is 0.265nm, and their D orbits overlap along the crystal axis. When electrons migrate from Fe2+ to Ti4+, Fe2+ is transformed into Fe3+, while Ti4+ is transformed into Ti3+, that is, Fe2++Ti4+→ Fe3+. In the process of charge transfer, the absorption energy of the spectrum is 2. 1eV, and the center of the absorption band is located at 588nm. As a result, only blue light is transmitted in the C-axis direction of sapphire, showing blue color (Figure 5- 12). When two octahedrons are connected by an edge in the direction perpendicular to the C-axis, the charge transfer absorption band slightly moves to the long wave direction, making sapphire appear blue-green in the extraordinary light direction.
Figure 5- 12 Source of sapphire color
There are two main reasons for the color of yellow and orange sapphires: one is the charge transfer between homonuclear atoms, and the other is the color center. There are iron absorption bands in the spectra of yellow and orange sapphires in Thailand and Australia, which proves that the coloring mechanism of these yellow sapphires may be related to iron ions. The transformation of Fe2+ into Fe3+ is accompanied by spectral absorption, resulting in yellow or orange. There are different explanations for the color of sapphire in Sri Lanka. The color of yellow or orange sapphire from this area is mainly due to the color of the center of the cavity. The bivalent cation replaces the trivalent aluminum ion in the corundum lattice, forming a position with insufficient positive charge-positive charge trap at the lattice position. In order to maintain electrical neutrality, divalent cations must be surrounded by corresponding monovalent cations or generate electron vacancies. When sapphire is heated under oxidizing conditions, electron vacancies are formed and monovalent cations are released to balance the missing monovalent cations. Therefore, divalent impurity ions and electron vacancies absorb visible light to produce yellow or orange.