Streak. It couldn't be hardness because some minerals are soft and metallic, and some are hard and metallic. Also, it can't be luster, because most gems are minerals, and most luster, but they're not metallic. And it can't be magnetism, because magnetite, the only magnetic mineral, is not really metallic.
According to this formula : ㏑[A] /[Ao] = - Kt when we have Ao = 0.3 m and K =0.46 s^-1 t = 20min = 0.2 x 60 =12 s So by substitution : ㏑[A] / 0.3 = - 0.46 * 12 ㏑[A] / 0.3 = - 5.52 by taking e^x for both side of the equation we can get [A] ∴[A] = 0.0012 mol dm^-3