"Key date" coins and ultra rare coins have in the past been excellent investments. A "key date" coin is generally considered to be the most important coin in a series, usually the lowest mintage and/or most expensive.

Rarity is simply the number of specimens extant of any particular item.

Key Dates and Rarities Index (3 Years).
Represents a 53% return over this three-year period.*

Source: Professional Coin Grading Service

A few famous examples of how rare coins can increase in value.*

1804 “Draped Bust” Dollar: the French Family specimen
1989 auction appearance            $242,000
2003 auction appearance            $1,207,500

1870-S “Seated Liberty” Half Dime
First appeared at public auction in 1980 for $425,000
Appeared at auction in 1985, realized $176,000
Sold at auction in 1986 for $253,000
Sold at auction in 2000 for $529,000

1885 Trade Dollar: the Worrell/Rudolph specimen
1993 auction appearance realized $242,000
2003 auction appearance realized $920,000

1913 “Liberty” Nickel: the Louis Eliasberg specimen
Sold at public auction in 1996 for $1,485,000
Sold privately 1/2004 for $3,000,000

      *Past returns not indicative of future results.