There are 91 sentences in the Book of Mormon that contain both "hard" and "heart." Notably in Alma 12, Alma-2 talks about hard hearts in 5 consecutive verses:
33 But God did call on men, in the name of his Son, (this being the plan of redemption which was laid) saying: If ye will repent, and harden not your hearts, then will I have mercy upon you, through mine Only Begotten Son;
34 Therefore, whosoever repenteth, and hardeneth not his heart, he shall have claim on mercy through mine Only Begotten Son, unto a remission of his sins; and these shall enter into my rest.
35 And whosoever will harden his heart and will do iniquity, behold, I swear in my wrath that he shall not enter into my rest.
36 And now, my brethren, behold I say unto you, that if ye will harden your hearts ye shall not enter into the rest of the Lord; therefore your iniquity provoketh him that he sendeth down his wrath upon you as in the first provocation, yea, according to his word in the last provocation as well as the first, to the everlasting destruction of your souls; therefore, according to his word, unto the last death, as well as the first.
37 And now, my brethren, seeing we know these things, and they are true, let us repent, and harden not our hearts, that we provoke not the Lord our God to pull down his wrath upon us in these his second commandments which he has given unto us; but let us enter into the rest of God, which is prepared according to his word.
Alma 12 and the first nine verses of Alma 13 were orignially part of the same chapter. It is interesting to me that Alma-2 admonishes his audience (a group of struggling-to-believe Nephite men) not to harden their hearts and then proceeds to discuss the "Holy Order" to which they are called in the following verses:
1 And again, my brethren, I would cite your minds forward to the time when the Lord God gave these commandments unto his children; and I would that ye should remember that the Lord God ordained priests, after his holy order, which was after the order of his Son, to teach these things unto the people.
2 And those priests were ordained after the order of his Son, in a manner that thereby the people might know in what manner to look forward to his Son for redemption.
3 And this is the manner after which they were ordained—being called and prepared from the foundation of the world according to the foreknowledge of God, on account of their exceeding faith and good works; in the first place being left to choose good or evil; therefore they having chosen good, and exercising exceedingly great faith, are called with a holy calling, yea, with that holy calling which was prepared with, and according to, a preparatory redemption for such.
4 And thus they have been called to this holy calling on account of their faith, while others would reject the Spirit of God on account of the hardness of their hearts and blindness of their minds, while, if it had not been for this they might have had as great privilege as their brethren.
5 Or in fine, in the first place they were on the same standing with their brethren; thus this holy calling being prepared from the foundation of the world for such as would not harden their hearts, being in and through the atonement of the Only Begotten Son, who was prepared—
6 And thus being called by this holy calling, and ordained unto the high priesthood of the holy order of God, to teach his commandments unto the children of men, that they also might enter into his rest—
7 This high priesthood being after the order of his Son, which order was from the foundation of the world; or in other words, being without beginning of days or end of years, being prepared from eternity to all eternity, according to his foreknowledge of all things—
8 Now they were ordained after this manner—being called with a holy calling, and ordained with a holy ordinance, and taking upon them the high priesthood of the holy order, which calling, and ordinance, and high priesthood, is without beginning or end—
9 Thus they become high priests forever, after the order of the Son, the Only Begotten of the Father, who is without beginning of days or end of years, who is full of grace, equity, and truth. And thus it is. Amen.