Why silver and not gold?

My son, if you receive my words,
And treasure my commands within you,
So that you incline your ear to wisdom,
And apply your heart to understanding;
Yes, if you cry out for discernment,
And lift up your voice for understanding,
If you seek for her as silver,
And search for her as hidden treasures;
Then you will understand the fear of the Lord,
And find the knowledge of God.

This passage addresses what must be done to acquire wisdom.  It is the fear of the Lord that is e beginning of wisdom.  My question is, "Why silver and not gold?"
Why does it say to seek silver rather than gold?  Hasn't gold always been more precious and valuable than silver?  The best answer I've ever heard to this question has to do with geological excavation.whereas gold may occasionally be found at the top of the earth, silver must always be mined- you have to dig for it, and you usually have to dig deep if you want to get silver.
You will have to dig, dig, dig into God's word to mine out the treasure of wisdom.

~Lou Priolo, Teach Them Diligently: How to Use the Scriptures in Child Training