In most of Ganesha’s idols, the left trunk of the Lord is found broken. There are many stories for this broken tusk.
The story in the Brahmanand Purana goes as this:
Parasurama beheaded the last one of his enemies and let out a shout of victory. He had just waged a war against Kartivirya Arjuna and the arrogant king’s entire army. And Parasurama had won. The burning revenge which had consumed him since he saw his father Jamadgni, lying dead in the mud with 21 scars all over his body, seemed to be slowly mitigating….Parasurama looked around tired as he saw the massacre around him and realized that none of his enemies were standing.
Parasurama fell down on the earth, feeling bone tired, as he glanced at the axe in his hands. He was called Parasurama – which meant Rama with the Axe. Parasurama considered this axe as the gift from Lord Shiva himself, who was also his teacher in martial arts and the arts of warfare. If anything else, it was Lord Shiva…Parasurama realized that he owed his entire victory to the three-eyed Lord.
Parasurama did not even blink, as he got up from the battleground and walked towards Mount Kailash, the image of Lord Shiva, almost drawing him like a magnet and the tiredness which he had felt, just vanished.
Very soon, the sage found himself just outside Kailash and was surprised when he found Ganesha, the son of Lord Shiva and Parvathi outside the door. Sage Parasurama was about to enter the house, when Ganesha stopped the man.
“You may not enter now!” Ganesha said sharply.
“Why?” Parasurama asked feeling completely bewildered.
“My parents are sleeping. I do not want anyone to disturb them, now.” Ganesha said firmly.
Parasurama looked at Ganesha, a tiny seed of anger building up. “I am Lord Shiva’s devotee and his student. I should be allowed to visit him anytime.” he said quietly.
“No!” Ganesha said with the same quietness and as one warrior would know another, Parasurama knew that Ganesha would not let him in….No matter what.
“Let me in or I will attack!” Parasurama said so quietly that Ganesha hardly heard him. But then Ganesha did not need to hear him.
Ganesha pulled up his weapon watching Parasurama. “No! I will not let you in.” Ganesha repeated.
And so the two men fought.
The two men matched each other, blade for blade, weapon for weapon. But Ganesha seemed like he was winning. Ganesha was more in control of himself and was able to fight wisely, observing Parasurama’s fighting technique.
Parasurama was however angry and he was getting more and more angry. He was being rash and could not believe that he was losing. In anger and desperation, Parasurama, hurled his axe at Ganesha.
For a second, Ganesha stood still. It took Ganesha less than the time for the weapon to leave Parasurama’s hands to realize that the axe was something which Parasurama had used on Lord Shiva, himself. (There are two versions on how Parasurama got the axe – according to one version, Lord Shiva challenged Parasurama to a fight and Parasurama was able to hurt Shiva with the axe and second version was that Lord Shiva presented the axe to Parasurama because of Parasurama’s extraordinary fighting prowess)
Ganesha realized that if he stopped the weapon, it would mean disrespect to his own father..
Ganesha closed his eyes, as he concentrated on the weapon.
The axe moved towards Ganesha and cut off the tusk of the elephant God, as it landed on the ground with a loud thud, waking up everybody around.
This woke up the Lord Shiva and the Goddess Parvathi.
Goddess Parvathi came out of the house first and all she saw was an axe with her son’s tusk lying on the ground.
Parasurama was shocked, when he saw the beautiful Goddess Parvathi, morphing before his very eyes. The eyes of the Goddess which were generally loving and peaceful, changed. And standing before Parasurama was one of the most dangerous warriors, Parasurama had ever seen – Goddess Durga, the warrior Goddess.
“You hurt my son!” The Goddess thundered looking at him angrily, bringing the weapon in a sweeping ark, as Parasurama watched the Goddess shocked, words utterly failing him. “For this, I will cut off your arms!” the Goddess said viciously, pulling her weapons.
“MOTHER!” Ganesha shouted, trying to get his mother to see reason. “It was a battle and it was me who let....” the words of Ganesha, were in vain.
Goddess Durga was burning with anger and she was in no mood to listen to anyone.
“Parvathi!” Shiva said sharply, trying to get the attention of the Goddess. “Please just look at Parasurama for what he is.”
For a huge second Durga watched Shiva and then she turned to Parasurama still angry. Taking a deep breath to control herself the Goddess turned to watch Shiva.
“Parasurama is my student and he is also like your son.” Shiva said quietly. “See him as your son and forgive him.” Shiva said.
Parvathi turned to Parasurama and then slowly, almost imperceptibly, Parasurama sighed in relief, as the Goddess changed to her normal form.
Parasurama realized that he owed his life to his teacher and....Ganesha.
Parasurama looked at Ganesha and realized what made the elephant headed God so special. It was out of respect for his father, that Ganesha had lost his tusk to the axe and despite that, Ganesha had still asked his mother to spare Parasurama's life.
Parasurama bowed to the elephant headed God with utmost devotion.
Ganesha watched the warrior-sage with a smile, as Parasurama handed over his beloved axe to him.
“This is yours, my Lord. Please forgive me!” Parasurama said, as Ganesha laughed.
The other story for the loss of Ganesha’s tusk is from the Mahabharatha, which involves Ganesha himself taking the tusk out to write the Mahabharatha, because that was the only pen, Ganesha could find to match the speed at which Veda Vyasa narrated the Mahabharatha.
There is also another story about the broken tusk:
Ganesha was very happy. His devotees had given him plenty of sweets and he had taken as many as he could and stuffed himself with quiet a lot sweets as he was walking home.
Naturally Ganesha was traveling on his vehicle – the mouse, and the two of them scurried along, back home.
As they were passing along a dense forest basking under the bright moonlight, out of nowhere, came a huge snake.
Naturally, Ganesha mouse trembled and ran from there.
Ganesha did not even have time to soothe his pet, as he fell down hard on the ground, and the sweets which he was carrying split and scattered all over the ground. But Ganesha had a bigger problem. As Ganesha had fallen, his stomach was pierced.
Quickly, Ganesha pulled the snake with one hand and tied it as a belt around his stomach to prevent any further damage.
However Ganesha’s misadventure was something which was not completely unobserved.
The moon God – Chandra Deva saw this and unable to stop himself, laughed. Angrily, Ganesha pulled his tusk and threw it at the Moon God, breaking the moon into many pieces. Chandra Deva was shocked but Ganesha was not yet done. “You will always be dark!” Young Ganesha cursed the Moon God.
Finally with the intervention of Lord Shiva, Ganesha agreed to modify the curse and so came the fifteen days of growing moon and the fifteen days of the waning moon.
And as Ganesha plucked his tusk and threw it at the Moon,
it is believed as one of the reasons why Ganesha has only one tusk.