Ok, well here's my little theory of the limitations of Earth bending, based on the observations on the show.

First, lets throw out the idea that Earth bending is based on the state of matter, in this case earth bendings can move anything in the solid state of matter.
Now we know this is not true because not are earth benders not able to move solid ice, but can't bend purified metals.

So this leaves me to believe that i's not the state of matter that's the trigger, but the composed element (and I mean element by the ones on the periodic table).

So lets look at the crust of our own world to compare.
www.windows.ucar.edu/tour/link=/earth/geology/crust_elements.html%26edu=high

The two most common elements in the earth's crust are oxygen (46%) and silicon(27%).

So with this in mind, it would be easy to conclude that 'oxygen' is not the 'trigger element' for if earth benders can bend oxygen, then they would be able to bend oxygen gas, and earth benders can't bend air.
en.wikipedia.org/wiki/Oxygen

So that leaves the element silicon. It's most abundant form are silcates (particularly Silicon dioxide - SiO2).
Note: Silicon dioxide is made up of one silicon and two oxygen, which is almost consistent with the percentage compostion of the earth's crust (almost 2 oxygen for every silicon).

The most well known form of silcates is 'sand.' Silcates and other more complex silcon compounds are well distributed in throughout the earth's surface.

Silicon has the potential to form diamonds and minerals.
Emerald is a mineral with the chemical formula Be3Al2(SiO3)6::Cr
Note that there's silicon in emerald, therefore earthbenders can bend emerald.

But mant minerals are mainly based on aluminum oxides rather than silcon oxides.
So if earth benders only bend silicon, than they can't bedn ruby(Al2O3::Cr) or sapphire(Al2O3). But hey, maybe earth benders can bend aluminum as well. It is the third most common element in the earth's crust (about 8%)...and my theory of the airbender's limitations is their ability to bend to two compunds nitrogen gas (N2) and oxygen gas (O2).

So in my conclusion, Earth benders can only bend any compounds of silicon...and maybe aluminum. (This also leads me to believe that earth benders can also bend lava if enriched with silica).

To look up any information on any of the compounds or elements I have mentioned, simply wikipedia them.

to refute the statement above^^

The question on the first post is very direct on a subject that should be taken from a non-modern day science level. The show is not based upon numbers and today's knoweldge, but the olden days of science. When earth was considered earth and rock was the element not Silicon or Oxygen or some element.

Okay not the greatest refute, but here's my thoughts on what bending is:
I believe that it is the movement of chi and the connection of the human chi to the element in state.

Earth-benders have to be tough and as their mentality becomes head-strong so does the chi
Water-benders are flexible and adaptive and movements require grace which water benders cause
Firebenders are fueled by anger and when people are angery they become volatile like fire
Airbenders are beyond me sorry i do not have an explaination for airbenders seeing as there is only one airbender and most of his airbending is "move the hand really fast and air bending happens"

Okay moving back to a slightly modern science level. Diamonds are made from pressure beyond any human powers. If we had enough pressure to make Diamonds from carbon then we would have made diamonds ages ago (disregard the fact there are fake diamonds)

Okay if an earthbender managed to move his chi enough to compress carbon into a diamond then that would be ridiculously strainful considering the Earth itself makes them pretty far down into the earth to reach the pressure level required.

so bottomline i do not believe Earthbenders can create diamonds, but as for the other gems i am not sure on their composition

Haha, lol. I just realized that I accidentally said, "Can earthbenders build diamonds, gems, and other minerals such as these?" I meant to say, "Can earthbenders bend diamonds, gems, and other minerals such as these?" That is my unfortunately, unnoticed, grammartical error. I am sorry that I caused you to go to such lengths Hieu. Thank you for the info though, Atmos.

mang -_-

this bites o well uh ill continue my lecture then
actually ill change and summerize.

You really can't base it off science, because the show has some pretty unrealistic qualities (exclude the obvious) We really do not know how the bending is designed. If it's based on science then Atmos has a good explaination, but if you base bending off the martial arts then it's about the Art of Chi and how to control it.

chi is the energy that flows through us by emotion, concentration, and personality.

I totally agree with Hieu on 'how' Earthbender's bend.

What I was trying to get at was "what" earthbender's bend.

I think it's possible to explain "anything" through a scientific approach...but that's of course my thoughtout opinion ^^.

Well in my thoughts since the majority of elements can be found as ore they can be bent in their raw forms...but then they have to do it by hand after that....gems...can be found in their raw states too..but after a certaint point (say shaping) the gem becomes unbendable and has to be altered by hand...Thats just my take on it...

Earthbending principles have been explained in various episodes, namely Return To Omashu and Bitter Work. Earth bending is connecting with the chi of the earth's crust, chi that is mostly still. The Earthbenders move the earth because they are connected to it. The reason they cannot bend metal is because it is artificially constructed. They can bend minerals and gems if they occur naturally, which they do.

I agree with the guy with the cool Eragon siggy. WHOOT! YAY SHRUIKAN!

Anyway, I think he makes a good point and I agree with him. I think earthbenders can bend stuff in the earth curst in its raw, natural form. Just wanted to say that.

On the season finale, as well as in the episode where they first meet Buumi (SP?) they are able to bend gems and crystals in the cave and also in the rock candy thingy. lol

They should be able to bend it in the show but I'm not sure about for this site ;D

-Shikimori

So, they can bend naturally occuring gems, but not artificially created ones? Explain this then: There are naturally formed diamonds and there are artificial diamonds created in labs. Their atomic structures are identical and the only reason that a difference is seen between them is because those that are naturally formed are considered 'better' for some strange reason. But that's beside the point, since there is no true difference between the two. So, why could one be bended and the other couldn't?

As for creating gems, I don't see why it wouldn't be possible. They're able to bend coal just fine and since some earthbenders can move huge masses of land they could probably put that power towards compressing the piece of coal, thus heating it up and combining those two aspects to produce a diamond (at least near the center where the pressure and heat would be strongest). This principle could be carried on to other gems created by heat and pressure.