is a manganese silicate with formula Mn2+2Al3(SiO4)(Si2O7)(OH)3 and hardness of 3.
Here is mixed with milky white quartz; the black striae are pyrolusite. A quite rare mineral, and never seen facet.
From Graveglia Valley ( Genova ) Molinello Mine, Italy.
Size: 14.6x11.5x7.3 mm.
Weight: 7.70 ct.