After the death of his first wife, Janet, eldest daughter of Henry, first lord Methven, Colin married Agnes Keith, eldest daughter of William, fourth earl Marischal, and widow of the regent Moray. During the regency Moray had been entrusted with the custody of the queen's jewels, and his widow had thus come into possession of the famous diamond, the Great Harry as it was called, which had been given to Mary as a wedding present by her father-in-law, King Henry of France, and which she, on her demission, had bequeathed to the Scottish crown as a memorial of herself. After her second marriage the lady, at the insistence of James Douglas, 4th Earl of Morton, had been summoned to deliver up the jewels belonging to the queen, and for not doing so the Earl and Countess of Argyll were, 3 Feb. 1574, put to the horn (Register of the Privy Council, ii. 330). The countess appealed to parliament, and even sought the intervention of Elizabeth, but the result was that on 5 March 1575 the earl, in his own name and that of his wife, delivered up the jewels.
This caused a complete estrangement between Argyll and Morton, and other events soon happened to aggravate the quarrel. Argyll and Morton were at odds in politics and in the field from then on. Argyll was one of the jury who brought in a verdict against Morton, 1 June 1581, for the murder of Henry, Lord Darnley. Though he took part in the raid of Ruthven, at which the person of the king was seized by the protestant nobles, Argyll also joined the plot, 24 June 1583, for his restoration to liberty.
Source www.thepeerage.com from The Dictionary of National Biography, Oxford Univ. Press
http://onlinebooks.library.upenn.edu/webbin/metabook?id=dnb