An inorganic Compound 'X' on treatment with concentrated $H_2SO_4$ produces brown fumes and gives dark brown ring with $FeSO_4$ in presence of concentrated $H_2SO_4$. Also Compound 'X' gives precipitate 'Y', when its solution in dilute $HCl$ is treated with $H_2S$ gas. The precipitate 'Y' on treatment with concentrated $HNO_3$ followed by excess of $NH_4OH$ further gives deep blue coloured solution, Compound 'X' is :