Becky, it's a question similar to 'how long's a piece of string?'
But to me, the term relates to any bead that's remotely earthy. That could mean that it features striations or other markings that mimic stone, rock, dirt, you name it.
It could also pertain to a 'style' which is swirly, has metal elements, or not, and can be in any colours (single or multiple), but with some kind of treatment/reactions that change a look or surface from smooth and shiny, to something less so.
So, really I think it's to do with effects, to mimic nature somehow.
Hope it helps
I'd call this an 'organic' simply because I went for an effect similar to agate:
I'd also call this an organic:
Or this:
