Alma is citing a text that is no longer extant. It must be an old text because it is describing the creation of the interpreters that eventually find their way into Alma’s hands. The Lord provided the stone (here in the singular, elsewhere specifically two stones, such as Ether 3:23) in order to “shine forth in darkness unto light.” These stones are specifically prepared to interpret the writings of the Jaredites (Ether 3:24), and are presented to the Brother of Jared during his marvelous experience when he saw the finger of the Lord.
Linguistic: The word “Gazelem” is given here for the first time in the Book of Mormon. Reynolds and Sjodahl suggest an etymology for the word:
Gazelem is a name given to a servant of God. The word appears to have its roots in Gaz—a stone, and Aleim, a name of God as a revelator, or the interposer in the affairs of men. If this suggestion is correct, its roots admirably agree with its apparent meaning—a seer. (Reynolds and Sjodahl. Commentary on the Book of Mormon. Deseret Book, 1977, 4:162).
Later in the history of the church that term is applied to Joseph Smith in his role as an interpreter, so we have the possibility of the naming of the stone in Ether/Alma and the person later. McConkie and Millet suggest:
This may well be a play on words. Is Gazelem the seer stone or the servant? It is difficult to tell from the passage and depends very much on the placement of a comma in the sentence. Perhaps it could refer to both. It is interesting to note that when Jesus called Simon Peter to the ministry he said: “Thou art Simon the son of Jona: thou shalt be called Cephas, which is, by interpretation, a seer, or a stone” (JST, John 1:42). Though this name or title of Gazelem may be used in regard to any seer who utilizes seer stones, it seems in this instance to be a direct reference to Joseph Smith the Prophet. (Joseph Fielding McConkie and Robert L. Millet, Doctrinal Commentary on the Book of Mormon, 4 vols. [Salt Lake City: Bookcraft, 1987-1992], 3: 278.)
Historical: The last remaining point relevant to interpreters is their association with the name Urim and Thummim. We have this connection made explicitly in the Doctrine and Covenants:
Doctrine and Covenants 17:1
1 Behold, I say unto you, that you must rely upon my word, which if you do with full purpose of heart, you shall have a view of the plates, and also of the breastplate, the sword of Laban, the Urim and Thummim, which were given to the brother of Jared upon the mount, when he talked with the Lord face to face, and the miraculous directors which were given to Lehi while in the wilderness, on the borders of the Red Sea.
Of course the Urim and Thummim are well known from the Old Testament. Are the interpreters the same as the Urim and Thummim of Aaron? It is not likely. Our texts say that the Lord prepared them and gave them to the Brother of Jared, and they appear to have retained a meaning specifically linked to the Jaredites. Nevertheless, we find these Jaredite interpreters with an Old Testament name. This is most likely a naming by association rather than direct connection. Joseph likely noted the similarity in concept between the two sets of stones, and simply used the more familiar label to describe the Jaredite interpreters.
Textual: With this particular background, it is probable that the text that is being cited here is the record of Ether, but a section left untranslated by Moroni.