From BiblePortal Wikipedia

Hastings' Dictionary of the Bible [1]

Adamant is twice (  Ezekiel 3:9 ,   Zechariah 7:12 ) used in AV [Note: Authorized Version.] and RV [Note: Revised Version.] as tr. [Note: translate or translation.] of shâmir , which is elsewhere rendered either ‘brier’ (  Isaiah 5:6;   Isaiah 7:23-25;   Isaiah 9:18;   Isaiah 10:17;   Isaiah 27:4;   Isaiah 32:13 ) or ‘diamond’ (  Jeremiah 17:1 ). ‘Diamond,’ which arose from ‘adamant’ by a variety of spelling (‘adamant,’ or ‘adimant,’ then ‘diamant’ or ‘diamond’), has displaced ‘adamant’ as the name of the precious stone, ‘adamant’ being now used rhetorically to express extreme hardness.

Watson's Biblical & Theological Dictionary [2]

שמיר , ‘Αδαμας , Sir_16:16 . A stone of impenetrable hardness. Sometimes this name is given to the diamond; and so it is rendered,  Jeremiah 17:1 . But the Hebrew word rather means a very hard kind of stone, probably the smiris, which was also used for cutting, engraving, and polishing other hard stones and crystals. The word occurs also in   Ezekiel 3:9 , and  Zechariah 7:12 . In the former place the Lord says to the Prophet, "I have made thy forehead as an adamant, firmer than a rock; that is, endued thee with undaunted courage. In the latter, the hearts of wicked men are declared to be as adamant; neither broken by the threatenings and judgments of God, nor penetrated by his promises, invitations, and mercies. See Diamond .

Fausset's Bible Dictionary [3]

(the English mean "unconquerable".) Unusually hard stones, as the diamond, which is a corruption of the word adamant; Hebrew Shamir ; Greek Smiris . Probably the emery stone or the uncrystallized corundum ( Ezekiel 3:9). Image for firmness in resisting the adversaries of the truth of God ( Zechariah 7:12). Image of hard heartedness against the truth ( Jeremiah 17:1). The stylus pointed with it engraves deeper than the common iron; with such a pen is Jerusalem's sin marked. Its absence from the high priest's breast-plate was because it could not be engraven upon; or perhaps it had not been introduced at that early time. (See Diamond .)

Smith's Bible Dictionary [4]

Adamant. The translation of the Hebrew word, Shamir in  Ezekiel 3:9 and  Zechariah 7:12. In  Jeremiah 17:1, it is translated "diamond." In these three passages, the word is the representative of some stone of excessive hardness, and is used metaphorically. It is very probable that by Shamir is intended Emery, a variety of Corundum, a mineral inferior, only to the diamond in hardness.

People's Dictionary of the Bible [5]

Adamant. This word is found twice in our version,  Ezekiel 3:9;  Zechariah 7:12, in both eases used metaphorically to signify firmness of character and purpose. The original word occurs again in  Jeremiah 17:1, where it is translated "diamond," with which the writer's pen is said to be pointed. The term must signify some exceedingly hard stone; and diamond is the hardest we know.

Morrish Bible Dictionary [6]

shamir.  Ezekiel 3:9;  Zechariah 7:12 . Though once translated 'diamond,'  Jeremiah 17:1 , it is used symbolically of extreme hardness, 'harder than a flint.' The word is translated 'briars' and signifies any 'sharp point:' hence a diamond point, or anything hard.

American Tract Society Bible Dictionary [7]

A name anciently used for the diamond, the hardest of all minerals. It is used for cutting or writing upon glass and other hard substances,  Jeremiah 17:1 . It is also employed figuratively,  Ezekiel 3:9;  Zechariah 7:12 . Others supposed the smiris, or emery, to be meant.

Webster's Dictionary [8]

(1): (n.) A stone imagined by some to be of impenetrable hardness; a name given to the diamond and other substances of extreme hardness; but in modern mineralogy it has no technical signification. It is now a rhetorical or poetical name for the embodiment of impenetrable hardness.

(2): (n.) Lodestone; magnet.

King James Dictionary [9]

AD'AMANT, n. Gr. L. adamas a word of Celtic origin.

A very hard or impenetrable stone a name given to the diamond and other substances of extreme hardness. The name has often been given to the load stone but in modern mineralogy, it has no technical signification.

Easton's Bible Dictionary [10]

 Ezekiel 3:9 Zechariah 7:12 Jeremiah 17:1

Holman Bible Dictionary [11]

Minerals And Metals

International Standard Bible Encyclopedia [12]

ad´a - mant ( שׁמיר , shāmı̄r ( Ezekiel 3:9;  Zechariah 7:12 )): In the passages cited and in  Jeremiah 17:1 , where it is rendered "diamond" the word shamir evidently refers to a hard stone. The word adamant ("unconquerable") is used in the early Greek writers for a hard metal, perhaps steel, later for a metal-like gold and later for the diamond. The Hebrew shāmı̄r , the Greek adamas (from which word "diamond" as well as "adamant" is derived) and the English adamant occur regularly in figurative expressions. All three are equally indefinite. Adamant may therefore be considered a good translation for shāmı̄r , though the Septuagint does not use adamas in the passages cited. There is a possible etymological identification of shāmı̄r with the Greek smyris ( smēris or smiris ), emery, a granular form of corundum well known to the ancients and used by them for polishing and engraving precious stones. Corundum in all its forms, including the sapphire and ruby, is in the scale of hardness next to the diamond. In English Versions of the Bible  Isaiah 5:6;  Isaiah 7:23-25;  Isaiah 9:18;  Isaiah 10:17;  Isaiah 27:4;  Isaiah 32:13 , shāmı̄r is translated "brier". See also Stones , Precious .

Kitto's Popular Cyclopedia of Biblial Literature [13]

The word thus rendered is, in Hebrew, Shamir. It occurs in  Jeremiah 17:1;  Ezekiel 3:9;  Zechariah 7:12. The Septuagint in  Jeremiah 17:1, and the Vulgate in all these passages, take it for the diamond. The signification of the word, 'a sharp point,' countenances this interpretation, the diamond being for its hardness used in perforating and cutting other minerals. Indeed, this use of the shamir is distinctly alluded to in  Jeremiah 17:1, where the stylus pointed with it is distinguished from one of iron. The two other passages also favor this view by using it figuratively to express the hardness and obduracy of the Israelites. Our Authorized Version has 'diamond' in  Jeremiah 17:1, and 'adamant' in the other texts: but in the original the word is the same in all. Bochart, however, rejects the usual explanation, and conceives it to mean 'emery.' This is a calcined iron mixed with siliceous earth, occurring in livid scales of such hardness that in ancient times, as at present, it was used for polishing and engraving precious stones, diamonds excepted. Rosenmüller urges in favor of this notion that if the Hebrews had been acquainted with the diamond, and with the manner of working it, we should doubtless have found it among the stones of the high-priest's breastplate; and that, as the shamir was not one of the stones thus employed, therefore it was not the diamond. But to this it may be answered, that it was perhaps not used because it could not be engraved on, or was possibly not introduced until a later period.

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [14]

a term vaguely used to describe any very hard stone, and employed in the Auth. Vers. in  Ezekiel 3:9;  Zechariah 7:12, as the rendering of שָׁמַיר (Shamir'), elsewhere ( Jeremiah 17:1) rendered DIAMOND (q.v.). Ἀδάμας ,  Sirach 16:1-30;  Sirach 16:1-30, in some copies.