Why Stone Tablet?
"Keep my commands and live,
And my law as the apple of your eye. Bind them on your fingers;
Write them on the tablet of your heart."
Solomon exhorts his son to keep the lessons he is learning close to his heart and close to his mind. At one point in western culture, it was common to tell people to "tie a string around your finger," as a reminder of something. Since that would be easily noticed, and seen, it would keep whatever was to be remembered at the front of the person's mind. This is the same spirit in which Solomon speaks to his son. He does not want his son to forget his wise sayings.
We as believers should never forget God's Word. Meditating on the Word daily is a good way to keep it fresh in one's memory. When the Devil launches temptations at us, we can answer the temptations with Scripture, as Jesus did. When tempted, He told the Devil, "It is written…" (Matthew 4:4, 7, 10). If the Word is written on the tablet of a believer's heart, he will delight in the Word. He will love the Lord, and he will obey him (John 14:15).