"The crown," i. e., the glory of the wise man constitutes his wealth. He alone is truly rich even as he alone (compare Proverbs 14:18 note) is truly king.
The seeming tautology of the second clause is really its point. Turn "the foolishness of fools" as you will, it comes back to "foolishness" at last.

But the foolishness of fools is folly - The Targum reads, The honor of fools is folly. The fool, from his foolishness, produces acts of folly. This appears to be the meaning.

The crown of the wise is their riches,.... Riches being used by them to increase and improve their knowledge and wisdom, and for the good of men, are an honour to them, and give them credit and reputation among men of sense and goodness; see Ecclesiastes 7:11;
but the foolishness of fools is folly; mere folly, extreme folly, just the same as it was; riches make them never the wiser; yea, their folly is oftentimes made more manifest through the ill use they make of their riches; spending them in the gratification of their sinful lusts; and making no use of them for their own improvement in knowledge, or for the good of their fellow creatures. The Targum is,
"the glory of fools is their folly;''
and that is no other than their shame, and in which they glory; such fools are wicked men.

The riches of men of wisdom and piety enlarge their usefulness.

(Compare Proverbs 3:16).
foolishness . . . folly--Folly remains, or produces folly; it has no benefit.

24 It is a crown to the wise when they are rich;
But the folly of fools remains folly.
From Proverbs 12:4, 31; Proverbs 17:6, we see that עטרת חכמים is the predicate. Thus it is the riches of the wise of which it is said that they are a crown or an ornament to them. More than this is said, if with Hitzig we read, after the lxx, ערמם, their prudence, instead of עשׁרם. For then the meaning would be, that the wise need no other crown than that which they have in their prudence. But yet far more appropriately "riches" are called the crown of a wise man when they come to his wisdom; for it is truly thus that riches, when they are possessed along with wisdom, contribute not a little to heighten its influence and power, and not merely because they adorn in their appearance like a crown, or, as we say, surround as with a golden frame, but because they afford a variety of means and occasions for self-manifestation which are denied to the poor. By this interpretation of 24a, 24b comes out also into the light, without our requiring to correct the first אוּלת, or to render it in an unusual sense. The lxx and Syr. translate the first אולת by διατριβή (by a circumlocution), the Targ. by gloria, fame - we know not how they reach this. Schultens in his Com. renders: crassa opulentia elumbium crassities, but in his Animadversiones he combines the first אולת with the Arab. awwale, precedence, which Gesen. approves of. But although the meaning to be thick (properly coalescere) appertains to the verbal stem אול as well as the meaning to be before (Arab. âl, âwila, wâl), yet the Hebr. אוּלת always and everywhere means only folly,
(Note: Ewald's derivation of אויל from און = אוין, null, vain, is not much better than Heidenheim's from אולי: one who says "perhaps" = a sceptic, vid., p. 59, note.)
from the fundamental idea crassities (thickness). Hitzig's אוּלת (which denotes the consequence with which the fool invests himself) we do not accept, because this word is Hitzig's own invention. Rather לוית is to be expected: the crown with which fools adorn themselves is folly. But the sentence: the folly of fools is (and remains) folly (Symmachus, Jerome, Venet., Luther), needs the emendation as little as Proverbs 16:22, for, interpreted in connection with 24a, it denotes that while wisdom is adorned and raised up by riches, folly on the other hand remains, even when connected with riches, always the same, without being either thereby veiled or removed - on the contrary, the fool, when he is rich, exhibits his follies always more and more. C. B. Michaelis compares Lucian's simia est simia etiamsi aurea gestet insignia.

Riches - They are a singular advantage and ornament to them. But - As for rich fools, their folly is not cured, but made worse and more manifest by their riches.