The best sideshow in London this week is an auction. In a stand-alone event, Sotheby’s is offering the world’s largest uncut diamond. Pre-sale estimates stretch beyond $86 million based on recent auction activity. The bidding will read the pulse of the uber-rich. Russians, Chinese, and Arabs—logical buyers for showy knick-knacks—may be out of the market, given economic upheaval at home. Certainly whoever buys this rough crystal will do so for emotional, rather than financial gain. Demand for crown jewels is exhausted; the piece value of a cut stone is a gamble. Gemstone marketers obscure the fact that the long-term rate-of-return on jewelry-grade diamonds has barely outpaced inflation.

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