In general words, gemstones are a group of materials of high value used to make jewelry. In this way, gemstones include fine mineral crystals (such as ruby and emerald), certain rocks (such as lapis lazuli and opal) and even some organic materials (such as amber, jet, and pearl) where among these alternatives, fine mineral crystals are the top ones. It is common to differentiate diamond, ruby, sapphire and emerald from other gemstones by considering diamond, ruby, sapphire and emerald as precious stones and other gemstones as semi-precious ones. Beside what a gemstone is made of as the most important factor, four famous Cs: Color, Cut, Clarity and Carat are among usual factors in determination of the value of a gemstone (carat is a unit of mass equal to 200 mg and here its use indicates the mass or weight of gemstone).
Ruby, a kind of pink to red gemstones, is a variety of the mineral corundum (α-Al2O3). Corundum in its pure and flawless form is colorless but impurities can change this. In the case of rubies, presence of chromium impurity in the crystalline structure of corundum makes it red and distinguishable as ruby. Replacement of Al3+ cations by Cr3+ cations in the crystalline structure of corundum introduces new absorption lines which can give the gem a visible red color for a suitable amount of replacement like one percent.