As far as I know, such a sign does not occur in chart of the International Phonetic Alphabet. Schwa by nature can not be stressed; it’s an unstressed central sound.
This sign /:/ is used to show a tense sound:
Bead /bi:d/, dead /di:d/
Yes. Perk and pearl are two one-syllable words that have the same vowel.
/3:/ is a central vowel generally known as the “hesitation sound”.
The lip rounding is neutral.
The length of such a vowel is mostly due to the retroflection that occurs right after the central sound.
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