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The phrase twenty-four carats (twenty-four carat, when adjectival) is used figuratively of something or someone that is unalloyed, throughgoing, trustworthy.
The reference is to twenty-four-carat gold, i.e., solid gold with the complete purity of twenty-four parts out of twenty-four. The following explanations, for example, are from The Detroit Free Press (Detroit, Michigan, USA) of Wednesday 6th December 1899 [page 4, column 6]:
“Twenty-four Carats Fine.”
Twenty-four carat gold is all gold; twenty-two carat gold has twenty-two parts of gold, one of silver and one of copper; eighteen carat gold has eighteen parts of pure gold and three parts each of silver and copper in its composition: twelve carat gold is half gold, the remainder being made up of three and a half parts of silver and eight and a half parts of copper.
It seems that the figurative uses of the phrase twenty-four carat(s) originated in American English against the backdrop of the gold rushes, i.e., of the sudden migrations of large numbers of people to places believed to be rich in gold—particularly of the California Gold Rush (1848-1855).
—Cf., below, quotation 3, and the following from the Hartford Daily Courant (Hartford, Connecticut, USA) of Wednesday 6th December 1848 [page 2, column 2]:
The Gold Fever.
The California gold fever is approaching its crisis. We are told that the new region that has just become a part of our possessions, is El Dorado after all.—Thither is now setting a tide that will not cease its flow until either untold wealth is amassed, or extended beggary is secured. By a sudden and accidental discovery, the ground is represented to be one vast gold mine.—Gold is picked up in pure lumps, twenty-four carats fine. Soldiers are deserting their ranks, sailors their ships, and every body their employment, to speed to the region of the gold mines.
These are, in chronological order, the earliest figurative uses of the phrase twenty-four carat(s) that I have found:
1-: From the Rock River Pilot (Watertown, Wisconsin, USA) of Wednesday 2nd February 1848 [page 1, column 4]:
An Interesting Theme.—There are three sorts of folks in the world—yea, four: Those that know one thing—those that know everything—those that know nothing—and those that glory in it. The first are the hobby-riders—the one-idea men; the second are the quacks; the third are the believers in the second; and the fourth are unalloyed fools, twenty-four carats fine.
2-: From The North Carolinian (Fayetteville, North Carolina, USA) of Saturday 27th January 1849 [page 3, column 5]:
There are some great fools in this world! Fools of twenty-four carats.
3-: From the following advertisement, published in the New Jersey Herald, and Sussex County Democrat (Newton, New Jersey, USA) of Saturday 3rd March 1849 [page 3, column 3]—H. I. (or H. J.?) & J. L. Barlow were booksellers:
BOOKS!—ONCE MORE—GOLD! (NOT FROM CALIFORNIA)—That new placer in Branchville is now seen to be inexhaustible. Its products are after close assaying found to be 24 carats fine, and “a few more of the same sort left.” The gold from this mine is very cheaply obtained; and all that have worked it are highly satisfied with the proceeds. New veins are coming to light constantly, and we cordially invite all interested to come and dig
SCHOOL BOOKS,
Miscellaneous Books, and every thing usually found in mines of this kind are to be had here in profusion. No need of going to California while so much of the pure material can be obtained nearer home. Come on adventurers, one and all, and choose for yourselves; where there is so much riches one cannot go amiss.
H. I. & J. L. BARLOW.
Branchville, March 1, 1849.
4-: From Special Notices, published in The Northern Inquirer (Bradford, Vermont, USA) of Saturday 27th May 1854 [page 3, column 1]:
Notice!
The Whigs of West Fairlee are invited to meet at the usual place of holding Town elections, on Wednesday, the 31st of May, inst., at 5 o’clock, P. M., to choose a delegate or delegates to represent their opinions at the State Whig Convention to be holden at Rutland on the 7th day of June next. It is the opinion of the Town Committee that no person should be put in nomination for any State office who is not a Whig 24 carats fine. It is further the opinion of the Committee that the delegation from West Fairlee should be instructed to go “dead set” against any coalitions. Per order of Town Committee.
West Fairlee, Nay [sic] 23, 1854.
5-: From the Hannibal Tri-Weekly Messenger (Hannibal, Missouri, USA) of Saturday 20th November 1858 [page 3, column 1]—however, here, I am unsure of the precise meaning of the phrase twenty-four carat reality:
MARRIED.
In this city on Wednesday, the 17th inst., by Rev. W. C. Busby, Mr. Robert W. Carson, of Sonoma County, California, to Miss Mary E. McKinnie, of Hannibal.
Our friends have adopted the most suggestive mode to cultivate a close intimacy and identical interest between the Iron and Golden States *, and we believe it the most effective plan in urging immediate Railroad intercourse. If we must furnish those delving men with mining implements, and adorn their auriferous sands with Missouri flowers, we want quick and safe transport.
When we look at the little bright offering tendered with this notice, we are nervously affected, and trust that they may enjoy hopes as glowing as its disc, with a twenty-four carat reality.
* “the Iron and Golden States”: Missouri and California, respectively.
6-: From the Buffalo Daily Republic and Times (Buffalo, New York, USA) of Saturday 4th December 1858 [page 3, column 2]:
Pedestrianism.—Going a-foot this morning became a matter of extremely dangerous uncertainty. On the slight foothold of snow which lay on our sidewalks this morning, a thin rain commenced falling at ten o’clock, freezing as it fell. […] A young lady who was trying to keep up a first class reputation by sticking to the sidewalks, suddenly disappeared on the corner of Swan and Main streets. Passers-by noticed a pile of clothing lying there, but too much occupied with attending to their own centres of gravity, they scrambled on, nor dared to cast a look behind them. An old gentleman evidently a twenty-four carat philanthropist, after many attempts, preserving, however, his equilibrium throughout the trying scene, came across the street, and steadying himself with his cane, peered down into the pile of merino and marabouts that lumped up supinely in that dangerous latitude. “By heavens!” exclaimed he after a prolonged gaze, “a human being!”
