of the first water

The Temple of the Golden Pavilionを読んでいます。
Well let's see. How do you tell the type of woman who likes clubfooted men? As a rule, she's a beauty of the first water. She has a cool tapered nose. But there's something a little loose about her mouth...
上の文に出てきた "beauty of the first water" の意味は文脈から分かりますが、辞書を見ます。
・Oxford English Dictionary: (Typically of someone or something perceived as undesirable or annoying) extreme or unsurpassed of their kind: she was a bore of the first water
・McGraw-Hill Dictionary of American Idioms and Phrasal Verbs: Fig. of the finest quality. This is a very fine pearl—a pearl of the first water. Tom is of the first water—a true gentleman.
何故この様な表現が生まれたのか気になります。次の辞書に説明がありました。
・The American Heritage® Dictionary of Idioms: Of the finest quality, as in That was a play of the first water. This idiom refers to a grading system for diamonds for their color or luster (compared to the shininess of water). The system is no longer used but the term, used figuratively since the early 1800s, has survived it.
ダイアモンドの鑑定と関係があったとはね。