Idocrase: H(OH)3Ca12(Al,Fe)6(SiO4)10 
Also known as Vesuvianite, is a fascinating mineral found originally on the volcano, Mt Vesuvius, hence one of the names.
Etymology: from the greek and means "mixed form", an allusion to its crystals showing a mixture of other mineral forms.
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Indigolite: インディゴライト
It is the name given to the tourmalines with a blue color.
Iolite: (Mg,Fe)2Al4Si5O18 Silicate of aluminium and magnésium.
Etymology: from the Greek "Ios" = violet. Als known as "Cordiérite" in reference to the French mineralogist Pierre Louis Antoine Cordier. Sometimes Iolite is also named "Dichroïte", from the Greek name meaning "two colors".
That stone has a strong pleochroism. That’s mean that it showns different colors threw the different axis.
Its unusual blue-violet color is attractive and is compared to a light blue sapphire with a purplish tint. It is the reason that cordeirite is sometimes called "water sapphire."
One of the most notable characteristics of cordierite is its strong pleochroism, or color changing ability. When viewed from one direction, the crystal or gemstone may appear blue or blue-violet. But as the crystal or gemstone is rotated to another viewing direction, the color will appear yellowish gray to light blue.
The Iolite was used by the Vikings. That characteristic was used to check the point on the see, regarding the sun.
