To be worth something appears to translate into German only as:
etwas wert sein
However, to be worth it appears to translate into German as either:
es ist wert
es lohnt (sich)
Does that mean that the verb sich lohnen (or just lohnen) can’t be used for to be worth something?
There are two slightly different usages of etwas wert sein.
One is the very literal meaning of being worth [sth.], where the sth. would have to be replaced by a value or an object of equivalent value.
Dieses Buch ist 100 EUR wert.
Meine Tochter ist mir 200 Kamele wert.
(sorry for the latter example, couldn't come up with something better quickly)
There is also the more or less fixed expression
Das ist (schon) etwas wert.
It means whatever das refers to is in some way valuable or helpful; it can mean all kinds of things, like a major task accomplished in a project or having a good first serve in tennis.
The second expression you're looking at
Das ist es wert. / Es ist es wert.
Das lohnt sich.
means exactly what you found, it's worth it/the trouble. Das lohnt is colloquial in my opinion, maybe also a regionalism. Es ist wert is simply wrong or at least missing an object (Wen oder was ist es wert?).
To answer the question at the end of your post: sich lohnen means that whatever is the subject of that phrase is expected to give you something valuable in return.
Ein Studium an der Universität lohnt sich.
Eine Investition in Gold lohnt sich.
Ausreichend Schlaf vor der Prüfung lohnt sich.
Of course this implies that sth. is worth sth. (in the long run), but you would translate it into English as sth. is worth it.
"Etwas wert sein" is a tangible expression of x is worth some known quantity, such as 100 Euros.
"Es ist wert" is an intangible expression that "x is worth the trouble," or "X is worth it" for short. The "worth" is not specified, just understood.
I would use expressions with "lohnen" in the first sense, of tangible quantities.
