In 1866, the U.S. government introduced a new 5 Cent piece made of nickel as an alternative for the silver Half Dime. The two coins were issued side-by-side through 1873, after which the Half Dime was discontinued and the "Nickel" became the coin of the land. The new metal, because it was so hard, made it difficult to strike the coins, and the dies suffered frequent cracking. Part of the problem was the intricate reverse, where stars were squeezed tightly between small rays surrounding the large "5". Ultimately, the rays were removed from the dies mid-1867, but this did not solve the cracking problem entirely.
High grade examples of this first-year-of-issue are readily available in grades all the way up to MS66, where PCGS has graded over 50 examples. As already mentioned, collectors should seek out well-struck examples (to the extent they are available), and avoid examples with natural flaws or excessive and/or large carbon spots.