A gemstone is a mineral that has exceptional aesthetic qualities - a beautiful appearance, gloss and glitter, color, purity and transparency. Gems are naturally resistant to wear and are rare in nature.
The most precious gems include the diamond, ruby, sapphire and emerald. They are considered "real" gemstones, others are considered "semi-precious".
The diamond has a privileged position among gemstones - it is the only one to have its own system of colour evaluation, cleanliness and quality of cut.
The ruby, sapphire and emerald are the so-called "big three" coloured precious stones. The differ from others in the rarity of their nature, beauty, natural resistance and a significantly higher price.
En.wikipedia.org. (2016). Gemstone. [Online] Available at: https://en.wikipedia.org/wiki/Gemstone [Accessed 8 Dec. 2016].