Hi , maybe you mean Alif+Hamza (ا+ء) like سماء / صحراء .
You are right, both words are feminine nouns but we can't consider it as a feminine marker because there are some other masculine nouns having same end like ذكاء / غباء / غناء
But in case of Adjectives it is a real marker for feminine adjective like حمراء / زرقاء / صماء / while the same adjectives for masculine are أحمر / أزرق / أصم