GEMSTONE INSIGHTS: Top Ten World's Rarest Gems Tanzanite: Found only in the foothills of Mount Kilimanjaro in Northern Tanzania, this blue-purple stone is a highly prized color-shifting gem. Taaffeite: It is named for Australian gemologist Richar Taaffe, who discovered a cut and polished specimen of the stone in 1945. Only a handful of these previous stones have ever been found, making them a true collector’s gem. Black Opal: The rarest type of Opal, the national gemstone of Australia. Black Opal is also the most valuable gem of its kind. Benitoite: Found near the San Benito River in California, Benitoite is a blue to purple gemstone first discovered in 1907. Under UV lights, it glows a brilliant chalky blue. Red Beryl: Also called Scarlet Emerald or bixbite, Red Beryl has only been found in Utah and New Mexico and has been described as 1000 times more valuable than gold. Alexandrite: It is named for Tsar Alexander II of Russia and exceedingly rare gemstone. It is a color-changing ...