Schorl

Formula: NaFe₃Al₆(BO₃)₃Si₆O₁₈(OH)

Origin of name: probably from old German, meaning unknown

Crystal system: hexagonal

Crystal class: 3m

Twinning: rare on {10-10} or {40-40}

Unit cell: a = 16.10 Å, c = 7.14 Å

Colour: black, brownish black, bluish black

Diaphaneity: 

Luster: vitreous to resinous

Habit: crystals short to long prismatic, vertically striated, or acicular to fibrous, thin tablets rare, radiating groups, massive

Hardness: 7

Specific gravity: 3.10 to 3.25

Cleavage: 2; {11-20} and {10-11} indistinct

Tenacity: conchoidal to uneven, brittle

Streak: colourless

Synonyms/varieties:

Comments: member of the tourmaline group, series with dravite