Transmitted light
Strongly pleochroic in red (may be pink, yellow, orange or violet). Rare. Monoclinic. High to extremely high relief, 1 perfect cleavage. Long prisms, sections with 6 sides. In XPL, intense colors, masked by the red color of the mineral. 0-34º or parallel extinction. ES(+/-), B(+/-). Both images in PPL.
Strongly pleochroic in red, may be brown, greenish, reddish or yellow. Monoclinic. Medium relief. One perfect cleavage if not altered. In XPL, 3rd and 4th order colors, masked by the strong mineral color. Extinction paralell and mottled. ES(+), B(-) with 2V from 0 to 35º. It is a very common mineral. Both pictures in PPL.
Strong pleochroism in red (yellow, green, blue, black and brown also are possible). Trigonal. High relief, cleavage not visible, fractures perpendicular to elongation. Basal sections (without pleochroism) triangular or pseudo-hexagonal. Extinction paralell, ES(-), frequent zonation. U(-), can have 2V of 5º.
Strong orange colors, can be red-brown. Pleochroism of variable intensity. It forms spherulites or radiated fibrous aggregates. The structure resembles that of chalcedony. In XPL, the color is almost the same. It has high birefringence and parallel extinction.
Usually pleochroic in red-brown (in green is possible). Monoclinic. High relief, no visible cleavage. Anhedral or tabular grains. In XPL, colors up to 3rd order. Dark halos around the grains may occur. Twins are common. B(-/+). Similar to brown hornblende. Both images in PPL.
Weakly pleochroic in red-brown to yellow-brown. Trigonal. Very high relief, granular or acicular crystals. The color is almost the same in PPL and XPL (reminiscent of titanite). Parallel extinction, ES(+), possible twins. No zoning, no alteration, dark halos possible. U(+). Images in PPL.
Varied and well-defined colors: yellow, yellow-brown, brown, dark blue, pale green, black. Weak pleochroism possible, stronger if colors are more intense. Very high relief, parallel extinction, ES(-), U(-). Common accessory in magmatic and metamorphic rocks. Images in PPL.
CORUNDUM: high relief, paralell extinction, in XPL gray colors (sapphyre is blue/green), U(-).
