Answer is: the volume of this spinel crystal is 2.5 cm³.
m(spinel crystal) = 9.5 g; mass of the crystal.
d(spinel crystal) = 3.8 g/cm³; density of the crystal.
V(spinel crystal) = m(spinel crystal) ÷ d(spinel crystal).
V(spinel crystal) = 9.5 g ÷ 3.8 g/cm³.
V(spinel crystal) = 2.5 cm³; volume of the crystal.