Water You Know (Or Thought You Did)

In browsing through the March 24, 2008, issue of The New Yorker, trying to look as if I have risen above my modest station in life, I came across this sentence, discussing the Eliot Spitzer scandal, in a fine essay by Hendrik Hertzberg (ah, America's puritanism!):

"That's what Larry Craig and David Vitter, hypocrites of the first water, are doing; they have faded into the echoing hallways of the Capitol like guerrillas melting into a crowd of campesinos."

"Of the first water"?

I had always seen or heard the phrase "of the first order."

Turns out "of the first water" has a very cool, as in ice (as in diamonds), etymology, as reported in The Phrase Finder, a gem of a British website (which parenthetically does not enlighten us on "of the first order"):

First water - Of the

Meaning

Of the highest quality.

Origin

From the gem trade. The clarity of diamonds is assessed by their translucence; the more like water, the higher the quality. This comparison of diamonds with water dates back to at least the early 17th century, and Shakespeare alludes to it in Tymon of Athens, 1607. The 1753 edition of Chambers' Encyclopedia has this under an entry for 'Diamond':

"The first water in Diamonds means the greatest purity and perfection of their complexion, which ought to be that of the clearest drop of water. When Diamonds fall short of this perfection, they are said to be of the second or third water, &c. till the stone may be properly called a coloured one."