Who really takes the jewels?

The thief was Prince Arjun. The sequel to the original movie sets off with him being released from prison. More details can be found below (hidden because of spoilers, as such: hover over it to read the content).

Synopsis "RETURN OF THE JEWEL THIEF":

Jewel thief Prince Arjun is released from prison and he sets about taking revenge against Vinay Kumar, now a renowned jewllery collector and entrepreneur, who uncovered his crimes and was the cause of his being imprisoned. Kumar has arranged for an exhibition of the famous Kohinoor gem, taken from India by the British, and Arjun plans to steal the gem and frame Kumar. To this end he organises for his son to be the officer assisting in the security and for Sheetal to seduce Kumar and get access to the security blueprints. She succeeds but is killed by Arjun who believes she has double- crossed him. He then kidnaps the girlfriend of an expert jewel thief, Johnny, whom he blackmails into stealing the gem. However, Johnny is actually an undercover policeman and together with Kumar they devise a plan to use a fake gem to fool the thieves. However, confusion arises over which is the real and which the fake gem.

Source: http://www.bfi.org.uk/films-tv-people/4ce2b7e60eb12

Checking Wikipedia also confirms this per the plot summary:

The British Government agree to loan the priceless Kohinoor diamond for an exhibition to multi-billionaire Vinay Kumar, on posting the security of Vinay’s jewel collection worth Rs.500 billion. Despite top security the Kohinoor diamond gets stolen. The prime suspects are Vinay himself, as simultaneously his jewel collection posted as security goes missing; Police Commissioner Surya Dev Singh, the very last person to have supposedly handled the diamond as also being the son of Prince Arjun, the jewel thief from the original film; Con-man Johnny, who has acquired a reputation of being a jewel thief; Jukaso, a gangster of international repute; and Chief Minister Neelkanth who had, in the presence of Surya Dev Singh, kept the Kohinoor in his safe. With the Chief Minister and the Police Commissioner themselves suspects, no one really knows who the real criminal is.

Source: https://en.wikipedia.org/wiki/Return_of_Jewel_Thief