A "tesoretto" is a group of coins.
I've seen this expression used in English but I've also found a link with the Italian word "tesoretto" in inverted commas. I prefer "group of coins", mainly because this is what they are, no special hidden there...

Italian context:
1) "Fondo S. Antonio, abitato: tesoretto
di stateri d'argento (III sec. a. C.)" (this is a photo caption) http://www.clio.it/sr/vr/chora/terredivaste.html

2) "Dalla Canonica di Spezzano provengono pregevoli reperti ceramici della fine del XV secolo. A Spezzano, nella casa di Alfredo Rossi, è stato inoltre recuperato un tesoretto composto da 36 monete in lega d'argento, per lo più di Francesco I d'Este." http://www.archeobo.arti.beniculturali.it/mostre/castello_di... (with photograph)

Enlgish context:
1) ""The 'tesoretto' of the Torre Cives consists of five golden coins dating back to the Byzantine period: one of Leone I and the others of Basilisco. They have been found in 1956 by some workmen during the restoration of a non-asphalted road leading to the Torre"."
http://www.parks.it/parchi.canavese/Epun.html

2) "Floriani Squarciapino M. 1948, "Tesoretto di monete d'argento", NSc, 326 ff.
Comment: About a group of silver coins from the republican period and the first century AD, found in a bronze vessel in the foundations of the Terme delle Sei Colonne (IV,V,10-11). "
http://www.ostia-antica.org/biblio/keywords/key1076.htm

3) "Since coins do hold value, people pay attention to them and worry about their security. Of the coins that survive to us, many are found in hoards. A coin hoard can be defined as "a group of coins of whose discovery " for example, in a purse or other container " makes it clear that the hoard was deliberately buried in a group," (Crawford, 191). "
http://oldmoney.vassar.edu/papers/survival.html

4) "This is a small but interesting collection and includes a group of coins found in the bank of Kings Sedgemoor Drain (...)" http://www.cornucopia.org.uk/html/search/verb/GetRecord/3957

5) group of coins (photo)
http://www.winchestermuseumcollections.org.uk/archaeology/in...

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Note added at 5 days (2007-04-23 21:39:16 GMT) Post-grading
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Thank you Anthony. Buon lavoro!