thor merb:gem:list
Installed local gems:
~ ParseTree-3.0.2
~ RubyInline-3.8.1
~ ZenTest-3.11.0
~ abstract-1.0.0
~ addressable-1.0.4
~ data_objects-0.9.6
~ dm-aggregates-0.9.6
~ dm-core-0.9.6
~ dm-is-list-0.9.6
~ dm-is-tree-0.9.6
~ dm-migrations-0.9.6
~ dm-serializer-0.9.6
~ dm-timestamps-0.9.6
~ dm-types-0.9.6
~ dm-validations-0.9.6
~ erubis-2.6.2
~ extlib-0.9.8
~ hoe-1.8.2
~ image_science-1.1.3
~ json_pure-1.1.3
~ mailfactory-1.4.0
~ merb-action-args-1.0
~ merb-assets-1.0
~ merb-auth-core-1.0
~ merb-auth-more-1.0
~ merb-auth-slice-password-1.0
~ merb-cache-1.0
~ merb-core-1.0
~ merb-exceptions-1.0
~ merb-helpers-1.0
~ merb-mailer-1.0
~ merb-param-protection-1.0
~ merb-slices-1.0
~ merb_datamapper-1.0
~ mime-types-1.15
~ rack-0.4.0
~ rake-0.8.3
~ rspec-1.1.11
~ ruby2ruby-1.2.1
~ rubyforge-1.0.1
~ sexp_processor-3.0.0
~ thor-0.9.8
~ tlsmail-0.0.1
~ uuidtools-1.0.6

bin/merb -V

Loaded full RubyGems instead of MiniGems
A gem was possibly implicitly loaded from /merb/gems/gems/json_pure-1.1.3/lib/json/ext.rb:7
/merb/gems/gems/json_pure-1.1.3/lib/json/common.rb:60: warning: already initialized constant State
 ~ loading gem 'dm-types' ...
 ~ loading gem 'dm-validations' ...
 ~ loading gem 'dm-serializer' ...
 ~ loading gem 'dm-is-list' ...
 ~ loading gem 'dm-is-tree' ...
 ~ loading gem 'tlsmail' ...
 ~
 ~ FATAL: The gem image_science (>= 0, runtime), [] was not found
 ~
 ~ FRAMEWORK_ROOT = /merb/gems/gems/merb-core-1.0/lib
 ~
 ~ Gem::LoadError: Could not find RubyGem image_science (>= 0)
 ~
 ~ FRAMEWORK_ROOT/merb-core/core_ext/kernel.rb:85:in `load_dependency'
 ~ FRAMEWORK_ROOT/merb-core/bootloader.rb:405:in `load_dependencies'
 ~ FRAMEWORK_ROOT/merb-core/bootloader.rb:405:in `each'
 ~ FRAMEWORK_ROOT/merb-core/bootloader.rb:405:in `load_dependencies'
 ~ FRAMEWORK_ROOT/merb-core/bootloader.rb:393:in `run'
 ~ FRAMEWORK_ROOT/merb-core/bootloader.rb:99:in `run'
 ~ FRAMEWORK_ROOT/merb-core/server.rb:172:in `bootup'
 ~ FRAMEWORK_ROOT/merb-core/server.rb:42:in `start'
 ~ FRAMEWORK_ROOT/merb-core.rb:169:in `start'
 ~
 ~ FATAL: The file image_science was not found
 ~
 ~ FRAMEWORK_ROOT = /merb/gems/gems/merb-core-1.0/lib
 ~
 ~ LoadError: no such file to load -- image_science
 ~ FRAMEWORK_ROOT/merb-core/core_ext/kernel.rb:90:in `require'
 ~ FRAMEWORK_ROOT/merb-core/core_ext/kernel.rb:90:in `load_dependency'
 ~ FRAMEWORK_ROOT/merb-core/bootloader.rb:405:in `load_dependencies'
 ~ FRAMEWORK_ROOT/merb-core/bootloader.rb:405:in `each'
 ~ FRAMEWORK_ROOT/merb-core/bootloader.rb:405:in `load_dependencies'
 ~ FRAMEWORK_ROOT/merb-core/bootloader.rb:393:in `run'
 ~ FRAMEWORK_ROOT/merb-core/bootloader.rb:99:in `run'
 ~ FRAMEWORK_ROOT/merb-core/server.rb:172:in `bootup'
 ~ FRAMEWORK_ROOT/merb-core/server.rb:42:in `start'
 ~ FRAMEWORK_ROOT/merb-core.rb:169:in `start'

merb -V works fine and I don't have image_science installed system wide.