"His mom believes in special stones."
Translation:אמא שלו מאמינה באבנים מיוחדות.
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.