I keep seeing this idea tossed around the Wiki: "Since bone is mostly made of calcium, doesn't that mean Earthbenders could use Bending on a person's bones?" Finally, my curiosity was piqued, & I decided to examine the claim for myself. This was a lot of Googling, so I'd hate to see it get buried. Consequently, blog post.
So let's recap that argument:
P1: Earthbenders can control calcium.
P2: Bones are mostly made of calcium.
P3: Earthbenders can control earthen impurities in objects, like metal.
C: Earthbenders can control bones. We'll call this "Bonebending."
But are these things true? I see no reason to doubt 1, & 3 is a proven fact. But what about 2? How much of your bones are REALLY made of calcium? Well, apparently, that has a high degree of individual variance. But we're gonna make a conservative estimate based on some general rules of thumb that I found on the internet.
Update: Whether or not Earthbenders can bend calcium, it turns out that calcium rarely exists in pure form. That's not too much of a problem, provided calcium still forms a mineral of some kind in the bones. If it is broken apart & made into a protein, that's another strike against Bonebending.
Update II: Bones turn out to be mostly mineral, hence, the verdict must be revised.
Your skeleton=15% of your weight.
Calcium in your body=1200-1400 grams.
99% of calcium is found in bones.
1 pound=about 454 grams
Let's take a 150 pound person. This comes to about 68,100 grams.
Fifteen percent of which is 10,215.
Subtract 1400 from that & you still have 8815 grams to account for. Put another way, calcium makes about 7% of your bones by weight.
Let's not forget, barring Yakone's genes, it is obscenely difficult to use waterbending to control the 70% of water in the human body. So, relatively, I do not think that 7% is "a lot," let alone "most of the bone." Plus, we've been so focused on the question of how much calcium is in your body that we've ignored the fact that this is how much calcium is in the entire skeleton, while a Bonebender would have to focus on a specific bone in order to do any damage. Working on an individual bone, you'd have that 7% divided something like 200 times.
My final verdict: Bonebending cannot be performed by Earthbending the calcium in your bones.
Revision: Bonebending can, however, theoretically be performed on the mineral compound that calcium forms in your bones. Bending a bone should be possible. Whether or not it can be used on a bone that is still inside the person's body is the next question.
Feel free to share your own thoughts below.