What is the ruling on doubting purity from menstruation upon seeing dry discharge resembling sand, and is a woman considered pure if this discharge is not liquid?

A woman recognizes purity from menstruation by one of two signs: dryness, meaning the cessation of blood, yellowness, and turbidity, such that the cotton swab comes out clean; or the white discharge (al-Qassah al-Bayda'), which is a fluid resembling plaster. It is not necessary to wait for the white discharge if dryness has occurred. As for what you mentioned about the white discharge mixing with dry particles resembling sand, if they are brownish, then it is turbidity, and you must wait for dryness or the pure white discharge. However, if the particles are colorless and the cotton swab comes out clean, then this is the white discharge, and purity has been attained. It is advisable to consult a doctor if there are any infections.