Quick answer
Gold is common enough to be counted. Speech that actually knows something is the rarer find, and the verse prices it accordingly.
Read both lines
The first line piles up quantity, gold and an abundance of rubies, so the comparison is not luxury against poverty but plenty against scarcity. Then the second line names a single vessel, and the singular is the argument: rubies come by the handful, this does not. What the verse calls precious is not knowledge itself but the lips of knowledge, knowledge that made it into speech and reached somebody. The book has already said wisdom is better than rubies at 8:11. Here it narrows the claim to the moment wisdom gets said out loud, which is the point at which it becomes useful to anyone else.
Related: Proverbs 8:11 · Proverbs 3:15 · Proverbs 10:20 · Proverbs 16:16
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.