Silicate of aluminium and magnesium. Cordierite is named after French geologist P. Cordier It Is also known dichroite, from the Greek, meaning “two-colored, or iolite, from another Greek root, meaning “violet.” The Vikings apparently knew how to use this mineral for navigational purposes. Because of its strong pleochroism, its behavior in relation to the incident light indicated the direction of the sun on overcast days. For this reason, It has also been dubbed “Vikings' compass."
Crystal system Orthorhombic.
Appearance Cordierite occurs as short, multifaceted prismatic crystals. It is normally gray, light or dark blue, or violet, but can also be yellowish gray. It is not very lustrous, but when transparent has vitreous luster. Transparent crystals display very strong pleochroism from blue-gray to almost colorless or from violet to veryapale grayish yellow.
Physical properties It has a hardness of 7-7.5. The density varies between 2.53 and 2.65 g/cma (but for gem quality stones is usually between 2.57 and 2.61 g/cm3). The refractive indices are ca. nu 1.532, ny 1 .540, or sometimes na 1.540, ny 1.549.
Genesis lt is found scattered in silica and alumina -rich igneous rocks but mainly in larger crystals, in schists, and areas of contact metamorphism.
Occurrence lt is found mainly in Germany (Bavaria) Norway, Finland, Brazil, and Madagascar.
Cordierite (or water sapphire)
Gem quality cordierite has the same name (or rather names) as the mineral itself, but was in the past also known as water sapphire, partiilarly when the color was not very intense. This is perhapsits most familiar name in the trade.
Appearance The color may vary from quite a deep blue to violet blue, light blue or-grayish blue but it always has very strong pleochroism being a much lighter gray or wan yellow in one direction." For obvious, aesthetic reasons, gems are cut so that this color is only visible from the side. The most common type of cutisthe rectangular, step type, not always with truncated corners. One also comes across cordierites with a cabochon cut or minutely engraved, particularly in the case of less transparent specimens with numerous inclusions. Most stones are a few carats in weight not too small, therefore, but never very large.
Distinctive features An essential characteristic of cordierite is its exceptional pleochroism, which may, however resemble certain tanzanites. Many cordierites have a decidedly cold, grayish coloration, whereas tanzanite is a warmer color, always with a hint of violet. Testing the density, which is very different, would remove any uncertainty.
Occurrence Cordierite comes mainly from the gem gravels of Sri Lanka, but also from the United States and Nanibia.
Value Rather low, that of minor gemstones.
Simulants and synthetics It is neither imitated nor produced synthetically.