While switching to the 1.8.7 Ruby version for a current project of mine I came over this weird error.
The iconv library ist missing if we use the json gem.
Surprisingly there are barely proper answers on google about it, so I though I would summarize the solution here.

The error again reproduced:

ruby-1.8.7-p352 :002 > require ‘rubygems’
 => true
ruby-1.8.7-p352 :003 > require ‘json’
LoadError: no such file to load — iconv
from /Users/dennamics/.rvm/rubies/ruby-1.8.7-p352/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:36:in `gem_original_require’
from /Users/dennamics/.rvm/rubies/ruby-1.8.7-p352/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:36:in `require’

The solution is to reinstall ruby and iconv using rvm like this, use your ruby version instead:
$ rvm pkg install iconv
$ rvm remove 1.9.2
$ rvm install 1.9.2 –with-iconv-dir=$rvm_path/usr

The original source for the fix ist the RVM Doc