Svetlana,
The Oxford English Dictionary suggest that the origin of carat is from an equivalent in 'grains', and as you say, one assumes carob 'grains'. See below :
[a. F. carat, ad. It. carato: cf. Sp. and Pg. quilate, earlier quirate, a. Arab. q{imac}r{amac}{tdotbl} (and qirr{amac}{tdotbl}) ‘weight of 4 grains’, acc. to Freytag ad. Gr. {kappa}{epsilon}{rho}{gaacu}{tau}{iota}{omicron}{nu} ‘little horn, fruit of carob or locust tree, a weight = of an obol’. Isidore (XVI. xxv. 10) has ‘ceratum oboli pars media est, habens siliquam unam et semis’; but originally the Gr. {kappa}{epsilon}{rho}{gaacu}{tau}{iota}{omicron}{nu} was identical with the L. siliqua, and was called the siliqua Græca. (Formerly confounded with caract mark, sign, character: see sense 4.)
As a measure of weight and fineness, the carat represents the Roman siliqua, as of the golden solidus of Constantine, which was of an ounce: hence the various values into which and enter or originally entered.]
{dag}1. The seed or ‘bean’ of the carob-tree. Obs.
1601 [1846]
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1601 HOLLAND Pliny I. 447 The fruit called Carobes or Caracts. [1846 LINDLEY Veg. Kingd. (1853) 550.]
2. A measure of weight used for diamonds and other precious stones, originally of an ounce, or 3 grains, but now equal to about of an ounce troy, or 3 grains, though varying slightly with time and place. It is divided into 4 carat-grains. Also attrib., as in a one-carat diamond.
1598
1575 1679
1667 1750
1743 1887
1875
1868
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1575 LANEHAM Let. (1871) 48 As for the valu, yoor iewellers by their Carrets let them cast, and they can. 1598 HAKLUYT Voy. II. I. 225 These pearles are prised according to the caracts which they weigh, euery caract is 4 graines. 1667 E. KING in Phil. Trans. II. 429 A Diamond of 10 Carats. 1679 Lond. Gaz. No. 1462/4 Lost..a parcel of Rough Diamonds..containing 38 Caracks . 1743 S. MADDEN Boulter's Mon. (1745) 57 Augmenting Carracts vastly raise Th' advancing Value of the Diamond's Blaze. 1750 BEAWES Lex Mercat. (1752) 873 The Jewellers divide the Ounce into 152 Parts, or Carats, and these into Grains, whereof four make a Carat. 1868 E. SEYD Bullion 146 Six carats are equal to 19 grains Troy weight. 1875 JEVONS Money (1878) 36 A one carat diamond. 1887 Whitaker's Alm. 362 The jewellery ounce is divided into 151 carats and 600 pearl-grains.
3. A proportional measure of one twenty-fourth used in stating the fineness of gold; e.g. if the mass contain 22 parts of pure gold and 2 of alloy, it is said to be 22 carats fine, or gold of 22 carats. Also attrib. with numeral, as 22-carat gold.
1575-6
1555 1676
1627 1722 1820
1806
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1555 EDEN Decades W. Ind. (Arb.) 211 The golde is of .xxii. caractes or better in fynesse. 1575-6 Act 18 Eliz. xv, No..Golde lesse in fynesse than that of xxij Carrottes. 1627 DONNE Serm. clvii. Wks. 1839 VI. 266 All their clods of Earth are Gold..of innumerable Carats. 1676 B. W[ILLIS] Man. Goldsm. 57 In his report of a Gold assay he [the Assay-master] sets it down by Carracts and Carract-grains. 1722 Lond. Gaz. No. 6059/2 The Gold..will be of the Standard of 22 Carrats. 1806 HUTTON Course Math. (1806) I. 129 It is said to be 22 caracts fine. 1820 G. CAREY Funds 94 The whole weight of any piece of pure gold is supposed to be divided into twenty four parts, called carats.
1581
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fig. 1581 SIDNEY Astr. & Stella xvi, Beauties which were of many carrets fine.
{dag}b. The tradition that the carat was originally a definite weight of gold, and = of some weight (see the etymology), survived in dictionaries, arithmetical works, etc., but these have erroneously taken it as of an ounce (= 1 scruple), and of a pound Troy (= oz.); also as of an ounce.
1558
1552 1686
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1552 HULOET, Scruple, a certayne measuer called a charect, whereof thre make a dramme. 1558 WARDE tr. Alexis' Secr. (1568) 49a, Take Damaskene Roses..an unce, Lignum Aloes, Galanga, Bengewyne, of eche of them a carette. 1656 BLOUNT Glossogr., Carat..is the third part of an ounce. 1667 E. CHAMBERLAYNE St. Gt. Brit. I. (1684) 12 The pound weight or 12 Ounces Troy of Gold, is divided into 24 parts which are called carrats so that each carrat is 10 penny weight troy, or half an ounce. 1686 W. HARRIS tr. Lemery's Chym. I. i. 75 A Carat of Gold is properly the weight of one Scruple. 1755 Gentl. Mag. XXV. 361 Weigh a small vial which holds about 4 ounces..fill it with water, and weigh that..1-128th of the whole..is called a caract. 1852 A. RYLAND Assay Gold & S. 28 The ounce containing 24 carats.
{dag}4. fig. Worth, value; estimate. Obs.
(Here a confusion with CARACT character, is evident.)
1598
1597 a1680
1650
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1597 SHAKES. 2 Hen. IV, IV. v. 162 Thou best of Gold, art worst of Gold. Other, lesse fine of Charract, is more precious. 1598 B. JONSON Ev. Man in Hum. III. iii. 22 No beautie, no; you are of too good caract, To be left so, without a guard. 1650 HOWELL Giraffi's Rev. Naples 125 Authority doth commonly discompose the mind of man, specially one of a base carat. a1680 BUTLER Rem. (1759) II. 14 Counterfeit Jewels of any Caract.