Verb
To change into a diphthong, as by affixing another vowel to a simple vowel.
Source: Webster's dictionaryThis vowel diphthongized in Germanic Source: Internet
Limburgish is typically a two-tone system, distinguishing between level high and falling, but the tones can be realized in other ways depending on syntax, and some vowels diphthongize or monophthongize under certain tones. Source: Internet
However, there are recognized situations in which other vowel pairs diphthongize. Source: Internet
There is also a tendency to diphthongize simple vowels, e.g., ū becomes eo or eu. Source: Internet