Installed ruby gem(NMatrix) not showing in irb - permissions

Question

I have installed ruby gem nmatrix with the command below:
sudo gem install nmatrix
I have installed it in ubuntu 12.04 . My gem env is as below:
RubyGems Environment:
- RUBYGEMS VERSION: 1.8.25
- RUBY VERSION: 1.9.3 (2013-05-15 patchlevel 429) [i686-linux]
- INSTALLATION DIRECTORY: /usr/local/rvm/gems/ruby-1.9.3-p429
- RUBY EXECUTABLE: /usr/local/rvm/rubies/ruby-1.9.3-p429/bin/ruby
- EXECUTABLE DIRECTORY: /usr/local/rvm/gems/ruby-1.9.3-p429/bin
- RUBYGEMS PLATFORMS:
- ruby
- x86-linux
- GEM PATHS:
- /usr/local/rvm/gems/ruby-1.9.3-p429
- /usr/local/rvm/gems/ruby-1.9.3-p429#global
- GEM CONFIGURATION:
- :update_sources => true
- :verbose => true
- :benchmark => false
- :backtrace => false
- :bulk_threshold => 1000
- :sources => ["http://gems.github.com/", "http://rubygems.org"]
- REMOTE SOURCES:
- http://gems.github.com/
- http://rubygems.org
Now when I go to irb and run code as below,to use NMatrix it gives error as :
irb(main):001:0> require 'rubygems'
=> false
irb(main):002:0> require 'nmatrix'
LoadError: cannot load such file -- nmatrix
from /usr/local/rvm/rubies/ruby-1.9.3-p429/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
from /usr/local/rvm/rubies/ruby-1.9.3-p429/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
from (irb):2
from /usr/local/rvm/rubies/ruby-1.9.3-p429/bin/irb:13:in `<main>'
irb(main):003:0>
So what should I do to use nMatrix gem I just installed. Also when i do
gem list
It is not showing nmatrix gem.
Doing
sudo gem list
only shows nmatrix gem.
Thanks.