Ores are a new addition to Animal Crossing: New Leaf. They can be found most often in the single breakable Rock that spawns in the town each day, but occasionally they can be found buried in the ground. Using a Silver Shovel on the Money Rock will sometimes produce gems instead of Bells.
They can be sold or used in custom furniture at Re-Tail. In addition, for 10,000 Bells, 3 Gold Nuggets can be brought to Cyrus to make a random piece from the Golden series.
An Island Tour involves collecting as many ores as possible in a certain amount of time, with each ore except the specified being worth 1 point. Each time a single ore is chosen to be worth three points. In some there is a secret ten point ore.
By giving a furniture piece to Cyrus combined with a crystal you can receive custom furniture that reflects the ore you give him.
Two rules of thumb can be seen here:
List of OreEdit