If the conjunction ו is followed by a noun that is stressed on the first syllable (this includes all monosyllabic nouns), the ו gets a קמץ, regardless of what vowel the first syllable of the following word has.
Hmm... I am not convinced by this proposed rule, because the examples cited, although correct, are exceptional.
In pairs like יוֹם
וָלַיְלָה and עֵץ
וָאֶבֶן, the conjunction is vocalised וָ because there is a particularly close relationship between the two elements joined together, thus also לֶחֶם
וָמַיִם, for instance. This is why it is so vocalised in תֹהוּ
וָבֹהוּ.
Otherwise, the vocalisation of ו is regular, thus:
עֲצֵי אַלְמֻגִּים הַרְבֵּה מְאֹד
וְאֶבֶן יְקָרָה
לֶחֶם לֹא אָכַלְתִּי
וּמַיִם לֹא שָׁתִיתִי
יֹום לְיֹום יַבִּיעַ אֹמֶר
וְלַיְלָה לְּלַיְלָה יְחַוֶּה־דָּעַת