My husband just brought home a handful of the first baby potatoes and one of them is as solid as a rock and won't cook. What caused this?

Solids, or starches is what defines a potato. While most potatoes are consistent in their solids chemistry, very occasionally will you discover an overachiever that made it to your kitchen. Why just one? Because whatever plant this particular potato originated from, its potato siblings were sorted and graded and ended up elsewhere in a packing facility. Remember that potatoes are indeed a living organism and subject to several slight annoying nuances (growth cracks, misshapen, etc.) as well as one that simply refuses to cooperate. Again, a rarity but it can occur.