A memorably harsh image that is regularly quoted at women, and rarely with the rest of what the book says about beauty.
Quick answer
The picture is of something valuable placed where it does not belong and does no good. Beauty without judgment is the mismatch being named.
Read both lines
The comparison is doing one specific thing: it is about incongruity, not about worthlessness. A gold ring is genuinely valuable, and the image does not deny that; it observes that the setting wastes it. The NKJV footnotes "discretion" as taste, so the missing quality is judgment, the capacity to tell what fits. This is the same complaint the book makes about any good thing paired with no sense, and its force comes from the deliberate crudeness of the picture. Nothing in the verse suggests the ring should not exist.
What it does not mean
This verse has been used as a general comment on women's appearance, and it is not one. Its subject is judgment, not beauty, and the same book that says this also writes an entire acrostic in honor of a woman's strength, enterprise, and speech, and casts Wisdom herself as a woman who builds a house and sets a table. A proverb about a mismatch is not a license to appraise anyone, and reading it as permission to do so requires ignoring almost everything else the book says. Note too what the parallel actually implies: the verse assumes the woman's worth. That is why the mismatch registers.
Related: Proverbs 31:10–31 · Proverbs 9:1 · Proverbs 31:30
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.