After hitting a rock, it broke into pieces and revealed a silver ore. I did this to another rock a couple of days later, and it revealed a ruby. This appears to be a new class of objects unique to New Leaf, but I'm not sure what they're for. Do these items have a purpose beyond being sold?
Ores/Gemstones can either be sold (for a tidy sum depending on rarity) or be used in custom furniture at Re-Tail (through Cyrus, once you unlock him).
To unlock Cyrus, you need to accomplish four requirements:
- Sell at least 100,000 Bells worth of items to Reese
- Own at least 100 different types of furniture (in your catalog)
- Own at least 50 different items of clothing (in your catalog).
- Be a resident of the town for over 7 days.
Afterwards, he should wake up and be be able to customize any of your furniture. Here, you can basically create your own furniture designs or use one of the default ones present in the game. I don't remember seeing restrictions on what ores and gemstones you can add, just on the amount. It's during this design process where you can include the ores/gemstones. Moreover, if you gave Cyrus a piece of furniture combined with a ore or gemstone, you can receive some unique furniture which displays the ore/gemstone's characteristics.