Is my strength the strength of stones? - That is, like a rampart or fortification made of stones, or like a craggy rock that can endure assaults made upon it. A rock will bear the beatings of the tempest, and resist the floods, but how can frail man do it? The idea of Job is, that he had no strength to bear up against these accumulated trials; that he was afraid that he should be left to sink under them, and to complain of God; and that his friends were not to wonder if his strength gave way, and he uttered the language of complaint.
Or is my flesh of brass? - Margin, "brazen." The comparison used here is not uncommon. So Cicero, Aca. Qu. iv. 31, says, Non enim est e saxo sculptus, ant e robore dolatus homo; habet corpus, habet animum; movetur mente, movetur sensibus: - "for man is not chiselled out of the rock, nor cut from a tree; he has a body, he has a soul; he is actuated by mind, he is swayed by senses." So Theocritus, in his description of Amycus, Idyll. xxii. 47:
Στήθεα δ ̓ ἐσφαίρωτο πελώρια και πλατὺ νῶτον,
Σαρκὶ σιδαρείῃ σφυρήλακος οἷα κολασσός.
Stēthea d' esfairōto pelōria kai platu nōton,
Sarki sidareiē sfurēlakos hoia kolossos.
Round as to his vast breast and broad back, and with iron flesh, he is as if a colossus formed with a hammer - So in Homer the expression frequently occurs - σιδήρειον ἦτορ sidēreion ētor - an iron heart - to denote courage. And so, according to Schultens, it has come to be a proverb, οὐκ ἀπὸ δρυὸς, οὐκ ἀπο πέτρης ouk apo druos, ouk apo petrēs - not from a tree, not from a rock. The meaning of Job is plain. He had flesh like others. His muscles, and nerves, and sinews, could not bear a constant force applied to them, as if they were made of brass or iron. They must give way; and he apprehended that he would sink under these sorrows, and be left to use language that might dishonor God. At all events, he felt that these great sorrows justified the strong expressions which he had already employed.

Is my strength the strength of stones? - I am neither a rock, nor is my flesh brass, that I can endure all these calamities. This is a proverbial saying, and exists in all countries. Cicero says, Non enim est e saxo sculptus, aut e Robore dolatus Homo; habet corpus, habet animum; movetur mente, movetur sensibus. "For man is not chiselled out of the rock, nor hewn out of the oak; he has a body, and he has a soul; the one is actuated by intellect, the other by the senses." Quaest. Acad. iv. 31. So Homer, where he represents Apollo urging the Trojans to attack the Greeks: -
Νεμεσησε δ' Απολλων,
Περγαμου εκκατιδων· Τρωεσσι δε κεκλετ' αυσας·
Ορνυσθ', ἱπποδαμοι Τρωες, μηδ' εικετε χαρμης
Αργειοις· επει ου σφιλιθος χρως, ουδε σιδηρος,
Χαλκον ανασχεσθαι ταμεσιχροα βαλλομενοισιν.
Illiad, lib. iv., ver. 507.
But Phoebus now from Ilion's towering height
Shines forth reveal'd, and animates the fight.
Trojans, be bold, and force to force oppose;
Your foaming steeds urge headlong on the foes!
Nor are their bodies rocks, nor ribb'd with steel;
Your weapons enter, and your strokes they feel.
Pope.
These are almost the same expressions as those in Job.

Is my strength the strength of stones?.... Is it like such especially which are foundation and corner stones that support a building? or like a stone pillar, that will bear a prodigious weight? no, it is not:
or is my flesh of brass? is it made of brass? or is it like to brass for hardness, or for sustaining any weight laid on it? it is not; and, therefore, it cannot bear up under the ponderous load of afflictions on it, but must sink and fail; it is but flesh and blood, and that flesh like grass, weak and feeble; and, therefore, death is better than life laden with such an insupportable burden.

Disease had so attacked him that his strength would need to be hard as a stone, and his flesh like brass, not to sink under it. But he has only flesh, like other men. It must, therefore, give way; so that the hope of restoration suggested by Eliphaz is vain (see on Job 5:11).

Is, &c. - I am not made of stone or brass, but of flesh and blood, as others are, therefore I am unable to endure these miseries longer, and can neither hope for. nor desire the continuance of my life.