Alexandrite is the rare color-change variety of the mineral chrysoberyl. Its rarity is a result of its unlikely chemical makeup. Alexandrite can only form when aluminum and beryllium combine with trace elements like iron, titanium and, most importantly, chromium. On rare occasion, vanadium may also be present. 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.