You can track the price appreciation of a range of luxury goods through various indexes. As an example, for paintings, antiques and other artwork, there's the Mei Moses Fine Art Index, which lists auction sales history from the major auction houses, such as Sotheby's and Christie's. Remember to study the provenance—the record of previous ownership—to gauge a piece of art's authenticity. Coins, stamps and all manner of other items have their own versions of these indexes, so you may want to do your homework.
None of these markets, of course, is as transparent, information-rich or liquid as the stock or bond markets.
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The right high-net-worth financial advisor can also help to parse your options. From an investment point of view, many advisors will give their blessing to a 5 percent or 10 percent portfolio allocation to the right luxury items.
Bear in mind that while every investment comes with fees, luxury items can be both expensive to buy and to keep. That million-dollar yacht, for instance, may cost $100,000 per year to staff, outfit and maintain. The same 10 percent rule of thumb goes for your aircraft.