An inorganic Compound ' $X$ ' on treatment with concentrated $\mathrm{H}_2 \mathrm{SO}_4$ produces brown fumes and gives dark brown ring with $\mathrm{FeSO}_4$ in presence of concentrated $\mathrm{H}_2 \mathrm{SO}_4$. Also Compound ' $X$ ' gives precipitate ' $Y$ ', when its solution in dilute HCl is treated with $\mathrm{H}_2 \mathrm{~S}$ gas. The precipitate ' Y ' on treatment with concentrated $\mathrm{HNO}_3$ followed by excess of $\mathrm{NH}_4 \mathrm{OH}$ further gives deep blue coloured solution, Compound ' $X$ ' is: