what is the gender of אבן, and if it is male, why is מיוחדות in the female form? are there cases where adjectives for male nouns are written in the female form?
It is female. The adjective always agrees with the noun, so if you see a feminine adjective, it means the the noun is feminine.
This is an exception to the rule the male nouns take -ים for plural – the plural of אבן is אבנים.
Why is it אמא and not האמא even though it is refering to a particular mom?
אִמָּא שֶׁלּוֹ מַאֲמִינָה בַּאֲבָנִים מְיֻוחָדוֹת
eema shelo ma'aminah b-avanim meyukhadot
audio for מיוחדות