Corundum is

Ferric sulphate on heating gives:

Which of the following substances will not give the borax bead test?

Fusion of $MnO_2$ with $KOH$ in the presence of air or an oxidizing agent like $KNO_3$ forms a colored compound. Identify the product and its color.

When an oxide of manganese $(A)$ is fused with $KOH$ in the presence of an oxidising agent and dissolved in water,it gives a dark green solution of compound $(B)$. Compound $(B)$ disproportionates in neutral or acidic solution to give purple compound $(C)$. An alkaline solution of compound $(C)$ oxidises potassium iodide solution to a compound $(D)$ and compound $(A)$ is also formed. Identify compounds $A$ to $D$ and also explain the reactions involved.

Which Lanthanoid compound is used as a pigment?