Many tests can authenticate a gem as a natural ruby. Beyond ensuring the stone has a deep red color (hence the name from the Latin "ruber," meaning red), trained gemologists may use various chemical and physical tests to prove that a stone is a genuine ruby.
Determination of Specific Gravity (SG)
Every gemstone has a specific gravity, which means the ratio of the gem's weight to the same volume of water. Natural rubies consist of a mineral called corundum that has a specific gravity of about 3.99 and, therefore, needs a special solution called "clerici," which is dense enough to float rubies and thereby help to identify them.
Determination of Refractive Index (RI)
Refraction of light is the slight bending of light rays as they move obliquely from one medium to another. The extent of this bending is \the refractive index, and every gemstone has its own range. Ruby tends to be between 1.76 and 1.77, which sets it apart from other stones such as tourmaline.
Microscopic Examination
A final test is scrutiny under a microscope. Rubies have irregularities and flaws within their crystal structure, which are called inclusions, and gemologists can spot these. Rubies tend to have straight, angular lines, which sometimes cause aster-ism, a shimmering star-like effect on the stone's surface when you move it.
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