Neela Thotha or copper sulphate is an inorganic compound whose chemical formula is CuSO4. Copper sulphate is found in the form of several compounds in which the quantity of crystalline water is different.

For example, its dry (non-existent) crystal is white or light yellow-green, whereas pentahydrate (CuSO4 · 5H2O) is bright blue.

Crystal water is present in blue stone and due to this crystal water, color is blue. 

When the blue stone is heated, the crystal water becomes free from which the color of the blue stone becomes white.