Answer:
I can be as thin as a picture frame but my insides have many things you can see.
These are the bumpy parts of your digits
What is often returned, but never borrowed?
What can be white and yellow at the same time?
What instrument plays from the heart?
I am the fountain from which no one can drink. For many I am considered a necessary link. Like gold to all I am sought for, but my continued death brings wealth for all to want more.