The most precious tanzanite gems are those with a strong-to-vivid blue, purplish blue, or violetish blue tint. These gems can be very small indeed! The rarest and most expensive tanzanite is ABERRANT, which has an intense indigo color that some sources claim is actually a form of purple. It weighs over 3 grams (0.11 oz), is at least 5.5 inches (14 cm) long, and has been valued at $750,000 to $1 million. Aberrant tanzanite was discovered in 1971 by John Hill in the Argyle Mining District near Vancouver, Canada. He was looking for gold when he found the gemstone, which is why it's also called "golden stone". Aberrants are extremely rare: only about 100 examples have been recorded in the world. They're kept in the British Museum and the Smithsonian Institution in Washington, D.C. Tanzanite is usually white or pale pink, but it can also be yellow, red, olive green, brown, black, or gray. The deeper the color, the more abundant the element niobium is in the stone.