Pearl is the only gemstone formed as a result of biological activities of living creatures. A pearl is used for making jewelry since ages due to its luminance and inner glow. It symbolizes beauty, love, happiness, and wealth. Purity and piety act as cornerstones when describing the property of pearls.
Alexandrite is the rare, phenomenal, color-change variety of the chrysoberyl mineral, along with its cousins yellowish transparent chrysoberyl and chatoyant or cat's-eye chrysoberyl. Alexandrite's rarity is a result of its chemical makeup; alexandrite can only form when aluminum and beryllium combine with trace elements like iron, titanium and, most importantly, chromium--which causes alexandrite's color change and is very rarely found in nature. The unlikelihood of the rare element chromium being in the right place to combine with aluminum and beryllium under exactly the right conditions to create alexandrite is what makes it so rare and valuable.