Gemstone value of the stone. Some treatments are used widely because the resulting gem is stable, while others are not accepted most commonly because the gem color is unstable and may revert to the original tone. Heat can improve gemstone color or clarity. The heating process has been well known to gem miners and cutters for centuries, and in many stone types heating is a common practice. Most citrine is made by heating amethyst, and partial heating with a strong gradient results in “ametrine” – a stone partly amethyst and partly citrine. Aquamarine is often heated to remove yellow tones, or
George Stone (bishop) tact, and qualities as a popular leader. Horace Walpole, who gives an unfavourable picture of his private character, acknowledges that Stone possessed "abilities seldom to be matched", and give him credit for charity and generosity; and he had the distinction of being mentioned by David Hume as one of the only two men of mark who had perceived merit in that author's "History of England" on its first appearance. He was himself the author of several volumes of sermons which were published during his lifetime. George Stone (bishop) George Stone (1708 – 19 December 1764) was the Church of Ireland