Ring in Fantasy and Cartoons

TV Series Supernatural

4 Horsemen Death, War, Pestilence and Famine uses their rings to expose their power

TV Series Vampire Diaries

Vampires wear a ring in order to be able to go out during the day

Movie Harry Potter

Marvolo Gaunt's Ring is one of the Horcruxes (Voldemort split his soul in 7 pieces and place those pieces in objects, so even if he dies, he would be able to return and those objects called Horcruxes)

Comics-Cartoon Green Lantern

Green Lantern wears a magic ring which carries a concentration of magic energy

Cartoon Tom and Jerry:The Magic Ring

Ring diffuse a spell everytime it tapped and it stucks on Jerry's head

Reference:

http://en.wikipedia.org/wiki/Magic_ring

Lord of the Rings - J.R.R. Tolkien

Lord of the Rings is a fantasy novel series written by Tolkien between 1937 and 1949. 
The trilogy consists the following books:
The Fellowship of the Ring
The Two Towers
The Return of the King

It's world's third best selling novel with over 150 million copies sales. There are some quotes which are very famous and known by every reader and everyone who has seen the movie:

“One Ring to rule them all, One Ring to find them,
One Ring to bring them all and in the darkness bind them. ” 
― J.R.R. Tolkien, The Fellowship of the Ring

“War must be, while we defend our lives against a destroyer who would devour all; but I do not love the bright sword for its sharpness, nor the arrow for its swiftness, nor the warrior for his glory. I love only that which they defend.” 
― J.R.R. Tolkien, The Two Towers

“The world is full enough of hurts and mischances without wars to multiply them.” 
― J.R.R. Tolkien, The Return of the King

First movie of the trilogy was on the theaters in 2001:
Director: Peter Jackson
http://www.imdb.com/title/tt0120737/

The Two Towers 2002
Director:Peter Jackson
http://www.imdb.com/title/tt0167261/
The Return of the King 2003
Director: Peter Jackson
http://www.imdb.com/title/tt0167260/


References:
http://en.wikipedia.org/wiki/The_Lord_of_the_Rings
http://www.toptenz.net/top-10-movies-better-than-the-book.php/lotr-book-covers
http://www.sodahead.com/entertainment/have-you-read-the-lord-of-the-rings-trilogy/question-2669219/?link=ibaf&q=Have%20You%20Read%20The%20Lord%20Of%20The%20Rings%20Trilogy&imgurl=http%3A//www.picgifs.com/wallpapers/movie-and-tv-shows/lord-of-the-rings/animaatjes-lord-of-the-rings-73676.jpg

One of the Deadly Sins: Greed

Among the deadly sins, greed can be related to ring, from some aspects.

“Earth provides enough to satisfy every man's needs, but not every man's greed.” 
― Mohandas Karamchand Gandhi

Greed can be explained as demanding more than one's needs. Dostoyevsky linked greed and envy as:

“The world says: "You have needs -- satisfy them. You have as much right as the rich and the mighty. Don't hesitate to satisfy your needs; indeed, expand your needs and demand more." This is the worldly doctrine of today. And they believe that this is freedom. The result for the rich is isolation and suicide, for the poor, envy and murder.” 
― Fyodor Dostoyevsky, The Brothers Karamazov

Firstly, size of the diamond on the ring became a way to show "richness" so it's a kind of show-off tool. It's not for meeting a need, but for display.
Small sizes of diamonds are never thought as "enough" and even the advertisements and campaigns are based on this thought. 

“Greed, envy, sloth, pride and gluttony: these are not vices anymore. No, these are marketing tools. Lust is our way of life. Envy is just a nudge towards another sale. Even in our relationships we consume each other, each of us looking for what we can get out of the other. Our appetites are often satisfied at the expense of those around us. In a dog-eat-dog world we lose part of our humanity.” 
― Jon Foreman

Secondly, another aspect can be manufacturers' greed. Sadly they do whatever it takes to obtain the diamonds. Blood Diamond is  a movie which is about this subject:

Blood Diamond 2006
Director: Edward Zwick
http://www.imdb.com/title/tt0450259/

[The scene depicts an R.U.F. labour mining camp where captive villagers are forced to mine for diamonds.]

Captain Poison: The Freetown government and their white masters have raped your land to feed their greed! R.U.F. have freed you!

[Armed guards are shown, supervising the captives.]

Captain Poison: No more slave and master here! R.U.F. is fighting for the people! R.U.F. is FIGHTING for Sierra Leone!

[One of the workers finds a diamond and hides it in his mouth, apparently successfully.]

Captain Poison: Any bastard think he would joke with me diamond, I go cut he throat! STOP!

[Captain Poison approaches the worker.]]

Captain Poison: Give it to me!

[The worker takes the diamond out of his mouth and hands it to Poison, who then shoots him.]

References:

http://www.goodreads.com/quotes/search?utf8=%E2%9C%93&q=greed&commit=Search

http://www.diamondsourceva.com/education/caratweight/caratweight.asp

http://en.wikiquote.org/wiki/Blood_Diamond