Sharp stones are under him - Margin, as in Hebrew, "pieces of pot sherd." The Hebrew word (חדוד chaddûd), means "sharp, pointed"; and the phrase used here means "the sharp points of a potsherd," or broken pieces of earthenware. The reference is, undoubtedly, to the scales of the animal, which were rough and pointed, like the broken pieces of earthenware. This description would not agree with the whale, and indeed will accord with no other animal so well as with the crocodile. The meaning is, that the under parts of his body, with which he rests upon the mire, are made up of sharp, pointed things, like broken pottery.
He spreadeth sharp pointed things upon the mire - That is, when he rests or stretches himself on the mud or slime of the bank of the river. The word used here and rendered "sharp pointed things" (חרוץ chârûts) means properly something "cut in;" then something sharpened or pointed; and is used to denote "a threshing sledge;" see this instrument described in Isaiah 28:27-28, note; Isaiah 41:15, note. It is not certain, however, that there is any allusion here to that instrument. It is rather to anything that is rough or pointed, and refers to the lower part of the animal as having this character. The Vulgate renders this, "Beneath him are the rays of the sun, and he reposeth on gold as on clay." Dr. Harris, Dr. Good, and Prof. Lee, suppose it refers to what the animal lies on, meaning that he lies on splinters of rock and broken stone with as much readiness and ease as if it were clay. But the above seems to me to be the true interpretation. It is that of Gesenius, Rosenmuller, and Umbreit. Grotius understands it as meaning that the weapons thrown at him lie around him like broken pieces of pottery.

Sharp stones are under him - So hard and impenetrable are his scales, that splinters of flint are the same to him as the softest reeds.

Sharp stones (i) [are] under him: he spreadeth sharp pointed things upon the mire.
(i) His skin is so hard that he lies with a great ease on the stones as in the mud.

Sharp stones are under him,.... And yet give him no pain nor uneasiness;
he spreadeth sharp pointed things upon the mire; and makes his bed of them and lies upon them; as sharp stones, as before, shells of fishes, broken pieces of darts, arrows, and javelins thrown at him, which fall around him: this does not so well agree with the crocodile, the skin of whose belly is soft and thin; wherefore dolphins plunge under it and cut it with a thorn, as Pliny (h) relates, or with spiny fins (i); but with the whale, which lies among hard rocks and sharp stones, and large cutting pieces of ice, as in the northern seas.
(h) Nat. Hist. l. 8. c. 25. (i) Sandys's Travels, l. 2. p. 78.

stones--rather, "potsherds," that is, the sharp and pointed scales on the belly, like broken pieces of pottery.
sharp-pointed things--rather, "a threshing instrument," but not on the fruits of the earth, but "on the mire"; irony. When he lies on the mire, he leaves the marks of his scales so imprinted on it, that one might fancy a threshing instrument with its sharp teeth had been drawn over it (Isaiah 28:27).

30 His under parts are the sharpest shards,
He spreadeth a threshing sledge upon the mire.
31 He maketh the deep foam like a caldron,
He maketh the sea like a pot of ointment.
32 He lighteth up the path behind him,
One taketh the water-flood for hoary hair.
33 Upon earth there is not his equal,
That is created without fear.
34 He looketh upon everything high,
He is the king over every proud beast.
Under it, or, תּחתּיו taken like תּחת, Job 41:11, as a virtual subject (vid., Job 28:5): its under parts are the most pointed or sharpest shards, i.e., it is furnished with exceedingly pointed scales. חדּוּד is the intensive form of חד (Arab. hadı̂d, sharpened = iron, p. 542, note), as חלּוּק, 1-Samuel 17:40, of חלק (smooth),
(Note: In Arabic also this substantival form is intensive, e.g., lebbûn, an exceedingly large kind of tile, dried in the open air, of which farm-yards are built, nearly eight times larger than the common tile, which is called libne (לבנח).)
and the combination חדּוּדי חרשׂ (equal the combination חדודי החרשׂים, comp. Job 30:6) is moreover superlative: in the domain of shards standing prominent as sharp ones, as Arab. chairu ummatin, the best people, prop. bon en fait de peuple (Ew. 313, c. Gramm. Arab. 532). lxx ἡ στρωμνὴ αὐτοῦ ὀβελίσκοι ὀξεῖς, by drawing ירפּד to Job 41:30, and so translating as though it were רפידתו (Arab. rifâde, stratum). The verb רפד (rafada), cogn. רבד, signifies sternere (Job 17:13), and then also culcire; what is predicated cannot be referred to the belly of the crocodile, the scales of which are smooth, but to the tail with its scales, which more or less strongly protrude, are edged round by a shallow cavity, and therefore are easily and sharply separated when pressed; and the meaning is, that when it presses its under side in the morass, it appears as though a threshing-sledge with its iron teeth had been driven across it.
The pictures in Job 41:31 are true to nature; Bartram, who saw two alligators fighting, says that their rapid passage was marked by the surface of the water as it were boiling. With מצוּלה, a whirlpool, abyss, depth (from צוּל = צלל, to hiss, clash; to whirl, surge), ים alternates; the Nile even in the present day is called bahr (sea) by the Beduins, and also compared, when it overflows its banks, to a sea. The observation that the animal diffuses a strong odour of musk, has perhaps its share in the figure of the pot of ointment (lxx ὥσπερ ἐξάλειπτρον, which Zwingli falsely translates spongia); a double gland in the tail furnishes the Egyptians and Americans their (pseudo) musk. In Job 41:32 the bright white trail that the crocodile leaves behind it on the surface of the water is intended; in Job 41:32 the figure is expressed which underlies the descriptions of the foaming sea with πολιός, canus, in the classic poets. שׂיבה, hoary hair, was to the ancients the most beautiful, most awe-inspiring whiteness. משׁלו, Job 41:33, understood by the Targ., Syr., Arab. version, and most moderns (e.g., Hahn: there is not on earth any mastery over it), according to Zac 9:10, is certainly, with lxx, Jeremiah., and Umbr., not to be understood differently from the Arab. mithlahu (its equal); whether it be an inflexion of משׁל, or what is more probable, of משׁל (comp. Job 17:6, where this nomen actionis signifies a proverb = word of derision, and התמשּׁל, to compare one's self, be equal, Job 30:19). על־עפר is also Hebr.-Arab.; the Arabic uses turbe, formed from turâb (vid., on Job 19:25), of the surface of the earth, and et-tarbâ-u as the name of the earth itself. העשׂוּ (for העשׂוּי, as צפוּ, Job 15:22, Cheth. = צפוּי, resolved from עשׂוּו, ‛asûw, 1-Samuel 25:18, Cheth.) is the confirmatory predicate of the logical subj. described in Job 41:33 as incomparable; and לבלי־חת (from חת, the a of which becomes i in inflexion), absque terrore (comp. Job 38:4), is virtually a nom. of the predicate: the created one (becomes) a terrorless one (a being that is terrified by nothing). Everything high, as the לבלי־חת, Job 41:33, is more exactly explained, it looketh upon, i.e., remains standing before it, without turning away affrighted; in short, it (the leviathan) is king over all the sons of pride, i.e., every beast of prey that proudly roams about (vid., on Job 28:8).

Stones - His skin is so impenetrable, that the sharpest stones or shells are as easy unto him as the mire.