To purify a precious metal into its most pristine form it must be refined. To do this, a metal is heated to the point of being in a liquid state. Once the ore is liquified, the dross (impurities) is removed and the refined metal cools back into a solid state, free from its previous flaws. Usually, this refining process occurs once. If, however, a flaw or impurity is found in the once-refined metal, the process is repeated. If done right, a metal which has been refined twice will be in a more pristine and purer state than it was after its first purification.
To help us understand the holiness of God's words, David compares His speech to silver which has been purified seven times. "The words of the Lord are pure words: as silver tried in a furnace of earth, purified seven times" (Ps. 12:6). The cost and expense of silver which has been refined seven times would be reflected in its value. It would be of elite value because of its elite purity.
The purity of God's word (i.e., the Scriptures) ought to make our esteem of it robust with awe and reverence (Ps. 119:161). How true are David's declarations in Psalm 19: "The law of the Lord is perfect... the testimony of the Lord is sure...The statutes of the Lord are right...the commandment of the Lord is pure...The fear of the Lord is clean...the judgments of the Lord are true and righteous altogether" (vv. 7-9). Therefore, "More to be desired are they than gold, yea, than much fine gold: sweeter also than honey and the honeycomb" (Ps. 19:10).
God's word is categorically trustworthy, because "the words of the Lord are pure words: as silver tried in a furnace of earth, purified seven times".