Kyanite is a source of material for the manufacture of highly refractory porcelains such as those used for spark plugs. See also Andalusite; Mineralogy; Silicate minerals; Sillimanite.

from rings-things.com:

Kyanite
aka Blue Talc, Sappare
A shimmery stone with unusual properties, kyanite can become the sparkle in a necklace or the spark plug in an engine. This aluminum silicate is a polymorph with andalusite and sillimanite, meaning it shares the same chemistry but a different crystal structure with these other minerals. Kyanite is commonly a deep blue, similar to sapphire, and for this reason is named from the Greek word kyanos for "dark blue." The stone also appears in green and black, and more rarely, yellow, white or gray. The color is not always uniform; it can be blotchy or streaky. The crystals are transparent to translucent, and are found in long blades or columns. Kyanite forms during regional metamorphism under elevated temperatures and pressures, in rocks such as gneiss and schist. It is also found in kimberlites — unusual intrusive rocks that contain diamonds. While most minerals have just one hardness, kyanite has a range of 4.5-6.5; the hardness varies depending on which way the stone is scratched (because kyanite's crystals are so long and thin). This gemstone has perfect cleavage lengthwise and good cleavage in a second direction. Therefore, kyanite is characterized by its blue color, differing hardnesses, perfect cleavage and bladed crystal form. Kyanite is seldom faceted due to difficulties in cutting and remains an unusual collector's gem.
Little historical information is available on kyanite, but it has been mined around the world for at least a century. At one time, French jewelers extensively used the name sappare when referring to kyanite; this was the result of an error by a mineralogist from Geneva. The mineralogist H.B. Saussure Jr. reportedly misread a label attached to a specimen thought to be sapphire. Despite recognition of the mistake, the name "sappare" became widespread, and that name is still occasionally heard when referring to kyanite. In addition to jewelry, kyanite is widely used for industrial applications, especially in the manufacture of spark plugs, electrical insulators and heat-resistant ceramic (to reduce shrinkage as it expands). Considered to be very powerful energetically and metaphysically, kyanite has been called "the tripping stone." It is a stone of channeling, altered states, vivid dreams, dream recall and visualizations, and is said to give protection during these states. Believed to bring loyalty, honesty and tranquility, kyanite is also attributed with diminishing anger and confusion. Notable occurrences of this stone exist in Brazil, the U.S.A. (North Carolina and Georgia), Switzerland, Russia, Serbia, India, Kenya, Myanmar and Australia.