Alexandrite is a rare gemstone that belongs to the chrysoberyl mineral family. It was first discovered in Ural Mountains of Russia in 1830 and was named after Tsar Alexander II. Alexandrite is known for its unique color-changing property, exhibiting green hues in natural daylight and appearing red or purplish-red under incandescent light.

The rarity of alexandrite can be attributed to its geological formation process. It forms in metamorphic rocks under specific temperature, pressure, and chemical conditions. These conditions are rarely found naturally, which makes the occurrence of alexandrite scarce.

Moreover, mining alexandrite requires extensive efforts as it is often found deep underground in small quantities. The extraction process involves drilling, blasting, and excavation techniques that require skilled labor and specialized equipment.

Another factor contributing to the rarity of alexandrite is the limited geographic locations where it can be found. Apart from Russia, other countries such as Brazil, Sri Lanka, Zimbabwe have also produced some amount of this precious stone; however, their production levels remain low compared to other gemstones.

Due to its scarcity and unique properties, alexandrite has become highly coveted among collectors and gem enthusiasts alike. Its value increases with size (carat weight) and quality (color intensity), making it one of the most expensive gemstones worldwide.

In conclusion, alexandrite’s rarity stems from its geological formation process that occurs under specific conditions rarely found naturally requiring extensive efforts for mining it due to being located deep underground in small quantities along with limited geographic locations where it can be found leading up to becoming one of the most expensive gems globally.