You have to keep in mind that the article in Spanish is used in certain way. There is no possibility in Spanish to omit the article, it must be there.
That said, Los huesos son blancos means the bones are white. There is no location given and indeed it's a generalization.... given the lack of context ;)
Generally speaking, the context tells you whether the English equivalent would have article or not. Remember that, you can also say esos/estos in spanish (those, these).
I believe that, in many cases, Duolingo is out of these subtleties and just provides sentences that can be translated word by word. Therefore, I would not over think it
Thanks, RAMOSRAUL. Can you please comment on an earlier sentence, "Los libros son hijos del cerebro" ? This is pretty obviously a generalization as opposed to a sentence about certain specific books, so the correct English translation omits the articles: "Books are children of the brain." But why did the Spanish lose the "los" before "hijos" ?
Maybe this is just one of those things that simply can't be explained. Equally, I think, is the fact that in the English, inserting "the" before "children" would be perfectly correct, but before "books" it would be wrong.
For general information, and for those wanting an alternative explanation why el hueso is in the Nature skill, here is a possible explanation - it is also the pit/stone of a fruit.
Even if bone was not intended to be in the Nature skill, the software may see the stone translation and pull sentences about bones into the skill.
There is also a verb deshuesar, to pit/stone a fruit. http://www.spanishdict.com/translate/deshuesar