Answer:
Taken from a mine and then locked up in a wooden case. I am used by students everyday.
When young I am sweet in the sun. When middle-aged I make you gay. When old I am valued more than ever.
I move without wings Between silken string I leave as you find My substance behind.
If you break me, I'll not stop working. If you can touch me, my work is done. What am I?
I can be repeated but rarely in the same way. I can't be changed but can be rewritten. I can be passed down but should not be forgotten.
Whoever made me don't want me. Whoever bought me don't need me. Whoever use me don't know me.