And when thou hast blowne thy nose, use not to open thy handkercheif, to glare uppon thy snot, as if yu hadst pearles and rubies fallen from thy braynes.

[Non si vuole anco, soffiato che tu ti sarai il naso, aprire il moccichino e guatarvi entro, come se perle o rubini ti dovessero esser discesi dal cielabro.]

Giovanni della Casa
Giovanni della Casa (1503-1556) Florentine poet, author, diplomat, bishop
Galateo: Or, A Treatise on Politeness and Delicacy of Manners [Il Galateo overo de’ costumi], ch. 3 (1558) [tr. Peterson (1576)]

(Source (Italian)). Alternate translations:

It is moreover extremely indecent [...], when you have blown your nose, to draw aside and examine the contents of your handkerchief; as if you expected pearls or rubies to distil from your brain.
[tr. Graves (1774)]

And when you have blown your nose you should not open your handkerchief and look inside, as if pearls or rubies might have descended from your brain.
[tr. Einsenbichler/Bartlett (1986)]