CategoryCyclosilicateFormula
(repeating unit)(Ca,K,Na, )(Al,Fe,Li,Mg,Mn)3(Al,Cr,Fe,V)6
(BO3)3(Si,Al,B)6O18(OH,F)4[1][2]IMA symbolTur[3]Crystal systemTrigonalCrystal classDitrigonal pyramidal (3m)
H-M symbol: (3m)Space groupR3m (no. 160)IdentificationColorMost commonly black, but can range from colorless to brown, red, orange, yellow, green, blue, violet, pink, or hues in-between. It can also be bi-colored, or even tri-colored. Rarely, it can be found as neon green or electric blue.Crystal habitParallel and elongated; acicular prisms, sometimes radiating; massive; scattered grains (in granite)CleavageIndistinctFractureUneven, small conchoidalTenacityBrittleMohs scale hardness7.0–7.5LusterVitreous, sometimes resinousStreakWhiteDiaphaneityTranslucent to opaqueSpecific gravity3.06+0.20–0.06[1]Density2.82–3.32Polish lusterVitreous[1]Optical propertiesDouble-refractive, uniaxial negative[1]Refractive indexnω = 1.635–1.675
nε = 1.610–1.650Birefringence−0.018 to −0.040; typically about −0.020 but in dark stones it may reach −0.040[1]Pleochroism

• Typically moderate to strong[1]

• Red: definite; dark red, light red

• Green: strong; dark green, yellow-green

• Brown: definite; dark brown, light brown

• Blue: strong; dark blue, light blue

Dispersion0.017[1]UltravioletfluorescencePink stones; inert to very weak red to violet in long and short wave[1]Absorption spectraStrong narrow band at 498 nm, and almost complete absorption of red down to 640 nm in blue and green stones; red and pink stones show lines at 458 and 451 nm, as well as a broad band in the green spectrum[1]