Solitaire is a game you can play all by yourself—all you need is a deck of cards. And just as solitaire is a game for one, a single diamond in a ring is also a solitaire.
Another name for the solitaire card game is patience. There are variations on this game, but most involve sorting cards into suits by ranking numbers (king, queen, jack, ten, etc.). The Latin root at the heart of this lonely word is solitarius, "alone or isolated." Originally, a solitaire was a widow, and eventually the word came to mean any solitary person. In the 18th century, the "gemstone" and "card game" meanings evolved.