There was a trend in the past of spelling semi-consonantal initial i (/j/) with a j:
- jeri / ieri
- jod / iod, yod
juta / iuta(loanword)- jota / iota
- jato / iato
- jella / iella
- jena / iena
- jodio / iodio
- jonico / ionico
- jattanza / iattanza
- jettare, jettatura, jettatore / iettare, iettatura, iettatore
and similarly inter-vocal i:
- aja / aia
- bujo / buio
- guajo / guaio
- notajo / notaio
- Savoja / Savoia
- alleluja / alleluia
- fidejussione / fideiussione
- pajo, paja / paio, paia
- ajola, ajuola / aiola, aiuola
- noja, annojare / noia, annoiare
- scojolo, scojattolo / scoiattolo
and additionally replacing plural suffix -ii (of -io) (to distinguish homophones from plural -i (of -o)):
- varj / varij, varii, varî, vari
- genj / genii, genî, geni
- occhj / occhii, occhi
- viaggj / viaggii, viaggi
- studj
- ufficj
- principj
- beccaj
but contemporarily, this orthography seems to have only been retained in a few proper nouns and latinisms:
- Jesi, Jonio, Jesolo, Ajaccio, Letojanni, l’Aja
- Jacopo, Jonio, Jole, Jannacci, Jolanda (Iolanda), Jago (Iago, Yago)
- Jacuzzi, Jovine, Jacobelli, Jacoviello, Jaja, Ojetti, Ajello, Scajola, Pistoj, Rejna
- Juventus, juventino (iuventino), juniores
and transcription of regional languages' words:
- ajo (Romanesco, cognate with aglio)
- naja (from Friulian naie, Old Venetian naia)
Apart from these proper nouns, are there any native Italian words still commonly spelled with a j?
Sources
• The Tuscan Master; Or a New and Easy Method of Acquiring a Perfect Knowledge of the Italian Language in a Short Time, Divided into Two Parts..., Dr. Marcello Guelfi Borzacchini
/i/,/i:/or/j/. I.e. not/dʒ/as in jazz, jeans, jeep, judo or/ʒ/as in abat-jour, déjà-vu (although this would catch 'false positives' like German influenced junghiano etc) – iacopo Mar 16 '18 at 23:16