Was Cain's offering unacceptable to the Lord because it was not an animal sacrifice without blemish?

What it doesn't say gives you a clue....did you notice the attitude when Cain's offering was rejected? God looks at the heart and knew his attitude wasn't right. Also Cain brought fruit....but not the first fruit (or the best of what he had) but Abel brought the firstlings...the young and the best. It was the offering of what was in their heart that God judged by.

Some point to the fact that Abel’s sacrifice was from the “firstlings of his flock,” whereas Cain’s was merely “some fruits of the ground.” But the problem was not in the quality of produce that Cain offered, for the account says that Jehovah looked with favor “upon Abel and his offering,” and with disfavor “upon Cain and upon his offering.” So Jehovah looked primarily at the heart condition of the worshiper. In doing so, what did he perceive? Hebrews 11:4 says that it was “by faith” Abel offered his sacrifice. So Cain apparently lacked the faith that made Abel’s sacrifice acceptable.

Abel made an offering to Jehovah God. Cain did likewise. Each brought of what he had: Abel, of the firstlings of his flocks; Cain, of his produce. They both had belief in God. They undoubtedly learned of Him from their parents and must have known why they all were outside the garden of Eden and denied entry to it. Their offerings indicated a recognition of their alienated state and of their desire for God’s favor. God expressed favor toward Abel’s offering but not Cain’s. How the approval and the rejection were manifested the record does not show, but it was undoubtedly evident to both men. The reason for God’s approval of only Abel’s offering is made clear by later writings. The apostle Paul lists Abel as the first man of faith, at Hebrews 11:4, and shows that this resulted in his sacrifice being of “greater worth” than Cain’s offering. By contrast, 1 John 3:11, 12 shows Cain’s heart attitude to have been bad; and his later rejection of God’s counsel and warning, as well as his premeditated murder of his brother Abel, demonstrated this.

By the same token, Cain was rejected, not because he offered the wrong kind of sacrifice, but because he lacked faith, as indicated by his actions. Jehovah had clearly pointed out to Cain: “If you turn to doing good, will there not be an exaltation?” (Genesis 4:7) God did not reject Cain on account of any displeasure over his offering. Rather, it was “because his own works were wicked”, marked by jealousy, hatred, and finally murder, that Cain was rejected by God.

It was because Cain didn't show his love and sincerity by giving the best of his produce, and because he wasn't sincere enough in the first place when he was told to make the offreing. God doesn't just look at a person's action but more importantly the intention and condition of one's heart in fulfilling obligations and responsibilities.

If you can recall the ground was cursed from Adam's transgression. The lamb was a sign of the coming of the lord who atoned for sin.

Gen 3: 17 And unto Adam he said, Because thou hast hearkened unto the voice of thy wife, and hast eaten of the tree, of which I commanded thee, saying, Thou shalt not eat of it: cursed is the ground for thy sake; in sorrow shalt thou eat of it all the days of thy life;

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