Sapphire is a gemstone variety of the mineral corundum, which is an aluminum oxide (Al2O3). Along with ruby, sapphire belongs to the corundum mineral species. The name "sapphire" is usually used to describe blue corundum, while other colors are referred to as "fancy sapphires."

Sapphires are typically found in igneous and metamorphic rocks that have undergone intense heat and pressure. They can also be found in alluvial deposits (riverbeds and ocean floors), where they have been transported by water and deposited in a new location.

The beautiful blue color of sapphire is caused by the presence of trace amounts of iron and titanium within the crystal lattice. In addition to blue, sapphires can come in a variety of colors including pink, yellow, green, purple, orange, and black. These colors are caused by different trace elements or structural defects within the crystal lattice.

Like rubies, sapphires are known for their durability, ranking 9 on the Mohs scale of hardness. This makes them ideal for use in jewelry, especially in settings that are exposed to wear and tear, such as rings.

One of the most interesting features of sapphires is their ability to display asterism, similar to rubies. When cut into a cabochon shape, sapphires can display a six-pointed star-shaped pattern called "star sapphire." This occurs when light reflects off of needle-like inclusions within the crystal, which align in a specific pattern.

Sapphire is a fascinating and beautiful mineral that has captivated people for centuries. Its unique colors, durability, and optical properties make it a highly desirable gemstone, and its geological backstory adds to its allure.

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