In the Rajput paintings, what was the purpose of burnishing the completed paintings with an agate?

To create a uniform sheen

Answer: To create a uniform sheen
Burnishing with an agate was done to lend the completed paintings a uniform sheen and an appealing resplendence.

Though Rajasthani and Pahari schools were separated by short distances, the pictorial styles that emerged and evolved in these kingdoms were significantly diverse in terms of either execution — fine or bold; preference of colours (brilliant or gentle); compositional elements (depiction of architecture, figures and nature); modes of narration; affinity for naturalism — or had emphasis on extreme mannerism. Paintings were painted on waslis — layered, thin sheets of handmade papers glued together to get the desired thickness. The outline was sketched on waslis in black or brown followed by colours fixed therein by brief notations or sample patches. Colour pigments were predominantly obtained from minerals and precious metals like gold and silver that were mixed with glue as the binding medium. Camel and squirrel hair were used in brushes. On completion, the painting was burnished with an agate to lend it a uniform sheen and an appealing resplendence.