Collectors say they collect out of passion not profits. But given the run-ups in prices for various collectible markets in recent years, it's hard not to think of art, wine and other collectibles as investments as well as sources of pleasure.
And some investments have done much better than others. According Knight Frank's Wealth Report, an index of the nine main collectibles markets grew by 175 percent over the past 10 years – a far better record than U.S. stocks. All nine categories tracked by Knight Frank increased in value except collectible furniture.
Classic cars were the top-performing collectible. Car prices surged 23 percent in the 12 months ending in the third quarter of 2012, the report said, and racked up gains of 11 percent over five years and 395 percent over 10 years.
(Read more: Vintage Cars Fetch $53 Million in a Weekend)
The big surprise was the number two collectible: coins. They don't get as much press as art or cars, but coins have racked up even better gains over the past year (25 percent), and 248 percent gains over 10 years.
Stamps came in a close third, with nine percent gains last year and 216 percent gains over 10 years. One of the kings of philately (stamp collecting) is Bill Gross, the bond king who put together one of the top collections in the world and has sold off some of his collection in recent years.
As Knight Frank says, however, "performance doesn't go hand in hand with popularity." Sometimes the most beloved collectibles are dogs as investments.
Art remains far and away the most widely collected collectible among the world's wealthy and affluent. The world's millionaires plan to increase their spending by 13 percent on art this year.
The second most popular collectible is watches – led by Asian collectors. That was followed by fine wine, jewelry and then cars.
With all collectibles, however, buyers should beware. "People often think these types of investments are more transparent and less complicated than traditional investments," cautioned Greg Davies of Barclays Wealth and Investment Management.
"In reality, they are generally less regulated and can be illiquid, expensive to trade and sometimes actually more difficult to understand unless you have a high level of expertise or inside knowledge."