Iolite

Iolite is the gem variety of cordierite, an aluminum magnesium silicate that forms during metamorphism of aluminum-rich sedimentary rocks. It requires specific pressure and temperature conditions (moderate pressure, moderate-to-high temperature) and is commonly found in gneisses, schists, and hornfels.

Iolite's most remarkable property is its extreme pleochroism - it shows three distinctly different colors depending on the viewing direction: violet-blue, blue-gray, and pale yellow-clear. This trichroism is stronger than in virtually any other gemstone. It's so pronounced that gem cutters must orient the stone very precisely to show the best blue face-up.

The Viking compass legend suggests that Norse seafarers used iolite as a polarizing filter to locate the sun on overcast days, enabling navigation across the open Atlantic. While the historical evidence is debated (calcite may be the actual 'sunstone' referenced in the sagas), iolite does have polarization properties and the story has become inseparable from the gem's marketing.

Iolite's dramatic pleochroism is its most diagnostic feature. Rotate the stone and watch it shift from violet-blue to pale yellow to gray-blue. No other common gem-quality blue stone shows this degree of color change with direction.

Distinguish from tanzanite (softer at 6.5, different pleochroism axis), blue sapphire (much harder at 9, different SG), and blue tourmaline (different crystal system, less extreme pleochroism). At hardness 7, iolite is durable but has some cleavage that requires care in setting.