A Japa Mala helps us to keep the count of mantras. Popularly the chanting is done in '108' and '27' times and there are Malas of as many beads as 108 and 27.
Rudraksha Malas are used in chanting for "Lord Shiva" mantras.
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Tulasi malas are for Lord Krishna. With all the other avatars of Lord Vishnu also
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Lotus Seed Mala is used for chanting mantras of Goddess Saraswati & Goddess Lakshmi.