The chastisement that befell Nebuchadnezzar was madness and banishment to live like a beast in the wilderness for seven years until he repented and worshipped God. 

The lesson we are to draw from this example is that even raw gold requires refinement before it can be used. God, in his wisdom, processes each of His elect. The Apostle Peter wrote: "Beloved, think it not strange concerning the fiery trial which is to try you, as though some strange thing happened unto you: But rejoice, inasmuch as ye are partakers of Christ's sufferings; that, when his glory shall be revealed, ye may be glad also with exceeding joy." (1 Peter 4:12-13)