Hi,
What are the differences between 'friable' , 'brittle' , 'crumbly' and 'frail' ?
In which contexts should we use them ? I mean with food...bones...figurative sense...
I'm not exactly sure what you mean by figurative in this context. Anyway, here are a few general comments.
'friable' I never hear this word spoken, and I very, very rarely read it.
'brittle' Food or bones that is/are easy to break. Dry, not wet.
'crumbly' Food that breaks easily into very small pieces. For bones, too, but only for long-dead bones.
'frail' Not for food. Bones that have no strength, perhaps due to age or disease.
Best wishes, Clive