Installation fails on Mac OS X - gem update

So, I ran "gem update" today and it was interrupted by this:

``` bash
Updating installed gems
Updating atomic
Building native extensions.  This could take a while...
ERROR:  While executing gem ... (Errno::EACCES)
    Permission denied - /Users/jessica/.rvm/gems/ruby-1.9.3-p448/extensions/x86_64-darwin-12/1.9.1/atomic-1.1.16
```

So, does anyone know what I should do to rectify this?
