Pure lithium is an extremely reactive metal, just like sodium or potassium. In its metallic form, it would react violently with water, which is why darkeyes doesn't have a chunk of lithium on her mantle. Lithium automatically forms ionic compounds, so it is found in nature in several rare salts. It exists mostly as a trace element, although it can occasionally be found in more concentrated forms. However, it's somewhat unlikely that the lithium in hot springs ever helped anyone: there wouldn't be nearly enough lithium absorbed through the skin. It was popular for a brief time in the middle of the 19th century as a panacea, and was the touted "ingredient" in many snake-oil cures, although they didn't actually contain any.

While lithium is a relatively dangerous drug, and there's a great deal of controversy over whether its clinical effects are actually subclinical metal poisoning, I'm pleased that it's been helpful for you, darkeyes.