One small but challenging set is the collection of matte proof Lincoln cents. Only nine coins to the set, but finding choice coins could take some time and a lot of searching.
Matte proofs were not popular with collectors, who favored the brilliant proofs. Many of these special coins were spent.
These coins are proofs, with the strong details, including Lincoln's hair and beard, his bow tie, and folds in his suit. Lines on the wheat ears are crisp and sharp. A grading guide, some years ago, showed a diagram of a squared edge, to enable collectors to tell a matte proof from an ordinary coin.
Matte proofs were wrapped in paper containing sulfur, which resulted in some lovely toning. Shades of copper, orange, warm red, even deep blue and purple have been seen. The background shimmers on well-preserved coins, making them desirable to any collector.
I once knew a collector who became enamored of these coins and put together a set. He liked the coins so much, he built another set!
All coins are pricey, especially the 1909 VDB, with a very low mintage.
Lincoln cent collectors who want something new, different, and genuinely scarce, will appreciate matte proof Lincoln cents.