How to find where gem files are installed

I can finds gems that are installed using gem list, but it doesn't show me where the gems are installed.

How can I find where the gems are, and how can I know before installing a gem where it will be installed?

Use gem environment to find out about your gem environment:

RubyGems Environment:
  - RUBYGEMS VERSION: 2.1.5
  - RUBY VERSION: 2.0.0 (2013-06-27 patchlevel 247) [x86_64-darwin12.4.0]
  - INSTALLATION DIRECTORY: /Users/ttm/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0
  - RUBY EXECUTABLE: /Users/ttm/.rbenv/versions/2.0.0-p247/bin/ruby
  - EXECUTABLE DIRECTORY: /Users/ttm/.rbenv/versions/2.0.0-p247/bin
  - SPEC CACHE DIRECTORY: /Users/ttm/.gem/specs
  - RUBYGEMS PLATFORMS:
    - ruby
    - x86_64-darwin-12
  - GEM PATHS:
     - /Users/ttm/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0
     - /Users/ttm/.gem/ruby/2.0.0
  - GEM CONFIGURATION:
     - :update_sources => true
     - :verbose => true
     - :backtrace => false
     - :bulk_threshold => 1000
  - REMOTE SOURCES:
     - https://rubygems.org/
  - SHELL PATH:
     - /Users/ttm/.rbenv/versions/2.0.0-p247/bin
     - /Users/ttm/.rbenv/libexec
     - /Users/ttm/.rbenv/plugins/ruby-build/bin
     - /Users/ttm/perl5/perlbrew/bin
     - /Users/ttm/perl5/perlbrew/perls/perl-5.18.1/bin
     - /Users/ttm/.pyenv/shims
     - /Users/ttm/.pyenv/bin
     - /Users/ttm/.rbenv/shims
     - /Users/ttm/.rbenv/bin
     - /Users/ttm/bin
     - /usr/local/mysql-5.6.12-osx10.7-x86_64/bin
     - /Users/ttm/libsmi/bin
     - /usr/local/bin
     - /usr/bin
     - /bin
     - /usr/sbin
     - /sbin
     - /usr/local/bin

Notice the two sections for:

• INSTALLATION DIRECTORY

• GEM PATHS

Stack Overflow Q#19072070, score: 546