Compare Proverbs 2:4. "Fine gold" is apparently a technical word of that commerce, the native gold in the nugget or the dust.

For the merchandise - סחר sachar, the traffic, the trade that is carried on by going through countries and provinces with such articles as they could carry on the backs of camels, etc.; from סחר sachar, to go about, traverse. Chaffarynge; Old MS. Bible.
And the gain thereof - תבואתה tebuathah, its produce; what is gained by the articles after all expenses are paid. The slaves, as we have already seen, got their liberty if they were so lucky as to find a diamond of so many carats' weight; he who finds wisdom - the knowledge and salvation of God - gets a greater prize; for he obtains the liberty of the Gospel, is adopted into the family of God, and made an heir according to the hope of an eternal life.

For the merchandise of it is better than the merchandise of silver,.... The believer is a spiritual merchant; faith is a trading with and for Christ, and for spiritual and heavenly things by him; and because there is a parting with something for Christ, as a man's sinful lusts and pleasures, his own righteousness, his friends and relations, when set in opposition to or competition with him, and even life itself, when called for; and because he runs a risk of suffering reproach, afflictions, and death itself; therefore this concern with him, and enjoyment of him, is called a "merchandise", which is "better than that of silver", or than silver which is got by merchandise: for Christ, and the things of Christ, are more valuable than silver, and to be preferred unto it; more useful and profitable than silver is, which a man may have a large abundance of, and lose his soul, whereas by Christ is the salvation of it; more satisfying than silver is, with which a man is never satisfied, whereas he that has Christ has enough, having all things; more pleasant in obtaining, and more safe in enjoying; a great deal of anxiety and vexation attend the one, and inexpressible pleasure the other; and more durable and lasting than that, the enjoyment of Christ is for ever;
and the gain thereof than fine gold; the doctrines and ordinances of Christ are more to be desired than gold, yea, than fine gold; the blessings of grace by Christ, such as redemption, pardon of sin, &c. are not obtained by corruptible things, as silver and gold, but are more precious than they; and even the graces of Christ in the hearts of his people, as faith, hope, and love, are more precious than gold that perisheth; and much more preferable must he himself be, and the gain that accrues to a believer by him, which is cent per cent an hundred fold, even in this world, and in the world to come everlasting life: it is all clear gain a believer gets by Christ and trading with him; he has him, and all with him, without money and without price; he has that which is more worth than the gain of the whole world, even the salvation of his immortal soul; the riches he has by Christ are immense and unsearchable, the riches of grace and glory; it cannot be said how great this gain is.

The figure of Proverbs 3:13 carried out.
it--that is, wisdom.
merchandise of silver--acquisition by trading.
fine gold--dug gold, solid as a nugget.