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n. A rare blue or light yellow to brown crystalline mineral, the rarest of three forms of titanium dioxide, TiO2, used as a pigment, especially in paint.

n. a blue or brown mineral, a form of titanium dioxide, used as a pigment

n. One of the three forms of native titanium dioxid; octahedrite. In color it is indigo-blue, reddish-brown, and yellow; it is usually crystallized in acute, elongated, pyramidal octahedrons.

"He said claims the ink was too recent because it contained a substance called anatase titanium dioxide could be rejected because medieval maps have been found with the same substance, which probably came from sand used to dry wet ink."

octahedrite