Rudraksha, also rudraksh, Sanskrit: rudrākṣa ("Rudra's eyes"), is a seed is traditionally used for prayer beads in Hinduism and Buddhism. The seed is produced by several species of large evergreen broad-leaved tree in the genus Elaeocarpus, with Elaeocarpus ganitrus being the principal species used in the making of organic jewellery or mala.
Rudraksha, being organic, is preferentially worn without contact with metal; thus on a cord or thong rather than a chain.
Usually the beads of Rudraksha are strung together as a mālā. Traditionally, it is believed that the number of beads is 108 plus one. The extra bead is the bindu. There must always be a bindu to the mala, otherwise the energy becomes cyclical and people who are sensitive may become dizzy. When the beads are stringed, it is best that they are strung with either a silk thread or a cotton thread. If you wear it with a thread then it is good to take care to change the thread every six months. Otherwise one day the thread may snap and the 108 beads will go all over the place. If the rudraksha is stringed with either copper, silver or gold also it is fine, but what happens most of the time is, it is done by a jeweler. When the jeweler ties a knot with a gold wire or whatever, usually they tie it too tight and close and the inside of the rudraksha cracks. It is very important to make sure that the strings are tied loose. It should not be tightened too close, because if the inside crumbles with the pressure, it is no good. The mala can be worn all the time. You can even wear it when you shower. If you take cold water baths and are not using any chemical soap, it is especially good for the water to flow over it and upon your body. But if you are using chemical soaps and warm water, it becomes brittle and will crack after sometime, so it is best to avoid wearing it at such times. 
For someone who is constantly on the move and who eats and sleeps in various places, rudraksha is a very good support because it creates a cocoon of your own energy. You might have noticed that when you go to a new place, sometimes you can fall asleep easily, whereas in certain other places you cannot fall asleep even if you are physically tired. This is because, if the situation around you is not conducive to your kind of energy, it will not let you settle down. For sadhus and sanyasis, places and situations could trouble them because they were constantly moving. One of the rules for them was never to put their head down in the same place twice. Today, once again, people have started eating and sleeping in different places because of their business or profession, so a rudraksha can be helpful.
Another thing is, sadhus or sanyasis living in the forest cannot drink water from just any pool because many times in nature, the water may be poisoned or contaminated with certain gases. If they were to drink it, it might cripple or even kill them. If a rudraksha is held above the water, if the water is good and drinkable, it will go clockwise. If it is poisoned, it will go anti-clockwise. It is also a way of checking the quality of food. If you hold it above any positive pranic substance, it will move in a clockwise direction. If you hold it over any negative pranic substance, it will move in anti-clockwise direction.
A shield against negative energies
It is also a kind of shield against negative energies. It is possible for some people to use negative energies to cause harm to someone else. This is a whole science by itself. One Veda, the Atharvana Veda, is all about how to use energies to your advantage and to someone else’s detriment. If somebody who has mastery over this wants to use it, so many things – extreme suffering and even death can be caused.
A rudraksha is a kind of shield against this. You may think nobody will do negative things to you, but it is not necessary that it should be targeted at you. Let us say somebody did it to the person sitting next you, but he is not receptive to it. Now you may get it because you are sitting next to him. It is just like when two people are shooting at each other on the street, they do not intend to shoot you, but you may get shot. Similarly these things also can happen. It is not intended for you, but it can happen if you are in the wrong place at the wrong moment. There need not be a huge fear about such things, but a mala is a kind of protection from that.
Naturally grown grooves, starting from the natural vertically or horizontally stalk point* reaching the opposite point, are termed MUKHI/Face. Any kind of artificial modification by any means to complete the natural incompletely grown MUKHI/face precludes the seed being known as a natural MUKH/Face.
Most rudraksha have a small opening* at the stalk point resulting from the extraction and cleaning process; which is further expanded by drilling to use the rudraksha for its benefits. (*this opening might be limited to the surface only or it might be present like a drill-hole).
Description of the tree
Elaeocarpus ganitrus grows in the area from the Gangetic plain in the foothills of the Himalayas to South-East Asia, Nepal, Indonesia, New Guinea to Australia, Guam, and Hawaii. Rudraksha seeds are covered by an outer husk of blue colour when fully ripe, and for this reason are also known as blueberry beads. The blue colour is not derived from pigment but is structural. It is an evergreen tree that grows quickly. The rudraksha tree starts bearing fruit in three to four years. As the tree matures, the roots buttress rising up narrowly near the trunk and radiating out along the surface of the ground.
Rudraksha beads are the material from which malas () are made. The term is used both for the berries themselves and as a term for the type of mala made from them. In this sense, a rudraksha is a rosary, used for repetitive prayer (japa), a common aid to worship in Hinduism and Buddhism. Rudrakshas also used for the treatment of various diseases in traditional Indian medicine.
Seeds show variation in the number of grooves on their surface, and are classified on the basis of the number of divisions they have. Different qualities are attributed to rudraksha based on the number of grooves, or "faces" that it has. A common type has five divisions, and these are considered to be symbolic of the five faces of Shiva. It can only be worn with a black or red string or, rarely, a gold chain.
Rudraksha malas have been used by Hindus and Buddhists as rosaries from at least the 10th century  for meditation purposes and to sanctify the mind, body and soul. The word rudraksha is derived from Rudra (Shiva—the Hindu god of all living creatures) and aksha (eyes). One Hindu legend says that once Lord Shiva opened His eyes after a long period yogic meditation, and because of extreme fulfillment He shed a tear. This single tear from Shiva’s eye grew into the rudraksha tree. It is believed that by wearing the rudraksha bead one will have the protection of Lord Shiva. The rudraksha fruit is blue in colour but turns black when dried. The central hard rudraksha uni-seed may have 1 to 21 faces.
Definition and meaning of the word Rudraksha
The word rudraksha is derived from two words - rudra (रुद्र) and aksha (अक्ष).
A. Aksha means eye. Rudra and aksha means the one who is capable of looking at and doing everything (for example, the third eye). Aksha also means axis. Since the eye can rotate on one axis, it too is known as aksha.
B. Rudra means the one who weeps. A (अ) means to receive and ksha (क्ष) means to give. Hence, aksha (अक्ष) denotes the ability to receive or give. Rudraksha is the one that has the ability to wipe our tears and provide happiness.
The rudra (rudhir, rudraksha) tree
A. Creation of the rudraksha tree from the tears of grief shed by Shankar (or Shiva) upon seeing the unrighteous conduct of demon Tarakasur’s sons, and their destruction by Shiva :
Through their righteous conduct and devotion unto Shiva, Tarakasur’s sons Tadinmali, Tarakaksh and Kamalaksh, attained divinity. After some time, seeing that they have returned to their original unrighteous conduct, Shankar was grief-stricken, and His eyes were filled with tears.
A few of these tears fell onto the earth; a tree sprang up from these, which came to be known as the rudraksha tree. Later, Shiva destroyed the sons of Tarakasur. - Gurudev Dr. Kateswamiji
B. General information on the rudraksha tree: found up to 3000m above, or at, sea level. The rudraksha tree grows in a narrow opening, not on open ground. Its leaves resemble those of tamarind or nux vomica, but are longer. It yields one to two thousand fruits annually. The Yatis (Ascetics) in the Himalayas survive only on these fruits. These fruits are also known as amrutphal (Fruits of Nectar). They satisfy thirst.
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- "Rudraksha - Everything you need to know about it".
- The translation of rudrākṣa as "Rudra's eyes" and definition as berries of Elaeocarpus ganitrus see: Stutley, p. 119.
- Stutley, M. (1985). The Illustrated Dictionary of Hindu Iconography. New Delhi, India: Munshiram Manoharlal Publishers. ISBN 81-215-1087-2.
- Koul, M. K. (2001-05-13). "Bond with the beads". Spectrum. India: The Tribune.
- Lee, D. W. (1991). "Ultrastructural Basis and Function of Iridescent Blue Color of Fruits in Elaeocarpus". Nature 349 (6306): 260–262. doi:10.1038/349260a0.
- 108 beads in number
- For use both to refer to the beads and to a mālā see: Apte, p. 804.
- Das, Subhamoy. "The Holy Rudraksha: Super Seed".
- For the five-division type as signifying Shiva's five faces and terminology pañcānana, see: Stutley, p. 119.
- Seetha, K. N. (2008). Power of Rudraksha (4th ed.). Mumbai, India: Jaico Publishing House. ISBN 978-81-7992-844-8.
- Laatsch, M. (2010). Rudraksha. Die Perlen der shivaitischen Gebetsschnur in altertümlichen und modernen Quellen. Munich: Akademische Verlagsgemeinschaft München. ISBN 978-3-89975-411-7.
- Source : Sanatan’s Holy text ''Shiva''