Gold.

A lemon. It’s a citrus fruit whose juice is acidic and can be used to conduct electricity for motors. A car or other purchase that has problems is known as a lemon and can be expensive to repair. Lemons have a tart flavor but do little harm (unless you get it in a cut, then it hurts like the dickens). The old saying goes, “When life gives you lemons, make lemonade” (I know, it’s spelled aid not ade but it sounds the same and wouldn’t make any sense the other way. It all works out nicely when the riddle is told instead of read.)

A seed. One seed turns into a plant or tree and seemingly disappears. When a plant dies, the seeds can be harvested to create new plants. From a single seed, hundreds more are created, vastly exceeding the original. Gardeners care for plants on bent knee and each new plant helps make the air fresher.