Answer:
I'm a precious stone in a pack of cards. What am I?
Who plays when he works and works when he plays?
I don't live within a house nor do I live without. Most will use me when they come in and again when they go out.
I am a shimmering field that reaches far. Yet I have no tracks and am crossed without paths.
I am just two and two. I am hot. I am cold. I am the parent of numbers that cannot be told. I am a gift beyond measure, a matter of course. What am I?
Whoever made me don't want me. Whoever bought me don't need me. Whoever use me don't know me.