The famous Hope diamond, once worn by Marie Antoinette, was bought tonight for over $300,000 by Ned McLean, son of John McLean. Mrs. McLean wore it as a head ornament, guarded by a former Secret Service man who in turn was guarded by two private detectives. Walnut-sized and exquisitely blue in color, the 44.5-carat stone stems from India. Its owners included Louis XIV, Louis XVI, and a London banker named Hope. A sinister history of ill luck, madness, and violent death clings to the unique gem.