From: Rabbi Moshe Sofer
To: Rabbi Yehonasan
The Chasam Sofer rules that the principle that black color indicates damaged red blood applies to both liquid and dry substances, so leniency based on black color alone is rejected. However, tiny dry particles that do not dissolve when rubbed with the fingernail and are unaccompanied by blood moisture may be permitted based on a combination of factors: their dust-like form resembles the Talmudic category of earth, their minuscule quantity indicates they are not of uterine origin, physicians confirm such particles commonly form in the kidneys, and the lenient positions of Tosfos, Rosh, and Ran do not require the Raavad's strict form-matching criterion.
Source: Shu"t Chasam Sofer YD §176