Purple sapphires have traces of chromium and iron, as well as titanium, and are available in many shades. The most precious blue sapphires are the ones that are colored blue, resulting from titanium being present at the time of stone formation. Not all sapphires are blue; the colors that we see are a result of impurities and a process called charge shifting.
Sapphires are usually formed when limestone and other metamorphic rocks are forced to crystallize under conditions of extreme heat. Moreover, these rocks usually need to contain large volumes of aluminum. Sapphires are formed during the process of aluminum-laden limestone crystallization.
In rubies and sapphires, color is explained by crystal field theory, but with sapphires, a slightly different process known as charge transfer produces the blue sapphires. The blue in a blue sapphire comes from the titanium mineral found inside the crystal. When the crystal of the corundum contains both iron and titanium, it produces beautiful blue sapphires.
If the transition microelement is either iron or titanium, then the corundum turns blue, and we have a blue sapphire. The presence of the following elements as a trace element in corundum produces green and yellow sapphires, and if mixed with titanium, produces blue sapphires. When iron is present in the chemical composition, sapphires have a yellow color. If the following elements are also included in the crystal structure, then the sapphire takes on a purplish-pink hue.
How Crystal Lattices Occur
When forming a crystal lattice, the color may change from white and clear to another hue if transition metal minerals are introduced. Different combinations of minerals may lead to a variety of colors of sapphire. A rare natural variety of sapphire known as color-changing sapphire displays a variety of colors under different lighting conditions.
The violet tint of the sapphire is formed by small amounts of vanadium; larger amounts will create a color-change sapphire. If chromium and iron are both present, violet and mauve sapphires will form (vanadium may also result in purple color, but this is quite uncommon in natural sapphires). Very high chromium concentrations create a ruby, while lower concentrations produce a pink sapphire.
While corundum needs to have at least 1 percent chromium before you can see a deeply red ruby, the sapphires blue is evident when there is just 0.01% titanium and iron.
When hues other than red are seen in the corundum, the gem is usually classified as a sapphire. Corundum crystals with gem-quality color in any color except red are called sapphires, and the red varieties are called rubies. Sapphire is any gemstone-quality corundum crystal that is not red (red corundum is known as rubies). All colors in the translucent form of the mineral corundum are known as sapphires.
How Sapphires Crystallize
Sapphires are crystals of the mineral corundum, made primarily from atoms of aluminum and oxygen in the 2:3 ratio (Al203). The composition of sapphire is aluminum and oxygen, formed into hexagonal, two-pyramidal shapes.
Sapphires may also be formed in recrystallized limestones and higher-grade metamorphic rocks, which are low in silica and rich in aluminum. Depending on where they came from, the qualities of sapphires can vary, as well as their incorporated content. Sapphires are formed below Earth’s surface, through tremendous pressure and intense heat, out of a mineral called corundum (aluminum oxide), that leaks out through fissures in igneous or metamorphic rocks.
Gemstone corundum is rarely mined from the metamorphic rocks that contain it. Mining rubies and sapphires The majority of gemstone-quality corundum occurs in metamorphic rocks, such as schist or gneiss; or igneous rocks, such as basalt or syenite. Basaltic, volcanic rocks of eastern Australia are the main sources of corundum, sapphires, and rubies, however, the presence of chromolithium is uncommon, and it is generally not possible to mine the chromolithium directly from the basaltic rocks.
Corundum, (sapphire and ruby) forms deep within Earth’s crust, being brought to the surface by igneous rocks with a high abundance of aluminum, but a low abundance of silicon, like basalt.
The Conditions that Create Sapphires and Rubies
Sapphire gems are thus created by specific changes in heat and pressure and may occur in metamorphic and igneous rocks alike. In most cases, a few Sapphire gems are formed 6-18 miles deep within Earth’s crust, when high pressures and temperatures over 800 degrees change sedimentary rocks (water deposits), like siltstones and shale.
Both rubies and sapphires owe their strong colors to impurities, the rubies due to the presence of chromium, while blue sapphires are due to both titanium and iron. Not surprisingly, rubies and sapphires are among the most sought-after gems, if combined with their awesome colors. Only the reds that are strongly colored are considered rubies, and lighter colored stones are known as pink sapphires. Rubies are red sapphires as they are part of the mineral species corundum, only in different varieties.
Rubies are a red form of corundum, while sapphires are a blue form, and Padparadschas are pinkish-orange forms, which are known by different names. The name sapphire is used for the form of corundum which varies in color from a very pale blue color to very dark blue. Chromium gives rubies their dark red color, but a little of it in the corundum crystal can produce instead a rare, precious pink-orange Padparadscha sapphire.
Other colors in sapphires are caused by different mixes and ratios of iron, titanium, and chromium. Sapphires contain trace amounts of iron, titanium, and nickel, giving them several colors including green, yellow, blue, orange, black, and pink.
These stones are fab alternatives to their expensive colored diamond cousins, though certain types of colored sapphires can also be pretty expensive. There are also colorless sapphires that may be used instead of diamonds because of the similarities between them and colorless diamonds. Although both sapphires and rubies are formed from similar mineral structures, sapphires generally possess better clarity.