Be sitting on a gold mine phrase
To possess something very valuable, especially without realizing it
Only when the valuer told me how precious this stone is, I realized that I was sitting on gold mine.
The land he inherited is very valuable, yet he doesn't know he is sitting on gold mine.
A proverb used to express that someone have strongly possession over something after a long time of using/keeping it. Others can not take it away from he/she, even if they have the legal rights to do so.