<Tr> <Th> </Th> <Td> Cyclosilicate </Td> </Tr> <Tr> <Th> Formula (repeating unit) </Th> <Td> (Ca, K, Na, ()) (Al, Fe, Li, Mg, Mn) (Al, Cr, Fe, V) (BO) (Si, Al, B) O (OH, F) </Td> </Tr> <Tr> <Th> Crystal system </Th> <Td> Trigonal </Td> </Tr> <Tr> <Th> Crystal class </Th> <Td> Ditrigonal pyramidal (3m) H-M symbol: (3m) </Td> </Tr>

Where does the best black tourmaline come from