Mala (rosary) used for counting (chanting) mantras dedicated to Shani Bhagvan is usually made Black Hakik stone (agate stone). Black is the color associated with Shani. Those suffering from bad position of Shani in horoscope and Sade Sati get relief by chanting Shani mantras using Mala made of Black Hakik stone.
Another mala that is used for chanting mantra is the mala made from Vajainti stone. This is also recommended for chanting Shani mantras.
Those people who follow and believe in astrology are of the view that for getting good benefit of mantras one should use malas associated with the particular deity.
Thus for Shani mantras the first choice is Black Hakik Stone mala.