Tourmaline

Tourmaline
A stone cut open and polished, revealing a bright rainbow of colors
General
CategoryCyclosilicate
Formula
(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 systemTrigonal
Crystal classDitrigonal pyramidal (3m)
H-M symbol: (3m)
Space groupR3m (no. 160)
Identification
ColorMost 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)
CleavageIndistinct
FractureUneven, small conchoidal
TenacityBrittle
Mohs scale hardness7.0–7.5
LusterVitreous, sometimes resinous
StreakWhite
DiaphaneityTranslucent to opaque
Specific gravity3.06+0.20–0.06[1]
Density2.82–3.32
Polish lusterVitreous[1]
Optical propertiesDouble-refractive, uniaxial negative[1]
Refractive indexnω = 1.635–1.675
nε = 1.610–1.650
Birefringence−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]
Ultraviolet fluorescencePink 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]
Main tourmaline producing countries

Tourmaline (/ˈtʊərməlɪn, -ˌln/ TOOR-mə-lin, -⁠leen) is a crystalline silicate mineral group in which boron is compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. This gemstone comes in a wide variety of colors.

The name is derived from the Sinhalese tōramalli (ටෝරමල්ලි), which refers to the carnelian gemstones.[4]

  1. ^ a b c d e f g h i GIA Gem Reference Guide. Gemological Institute of America. 1995. ISBN 0-87311-019-6.
  2. ^ "Tourmaline". Mindat.org. Archived from the original on 2005-12-28. Retrieved 2005-09-12. This website details specifically and clearly how the complicated chemical formula is structured.{{cite web}}: CS1 maint: postscript (link)
  3. ^ Warr, L. N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  4. ^ "tourmaline". Oxford Dictionaries. Archived from the original on October 16, 2021. Retrieved 2021-03-19.