Google the title will show two libs: Camellia and HornetsEyes both of them have are built in C/C++ and have complete Ruby interface.
Camellia 2.7.0 gem installation fail on Leopard. Then I compiled Camellia 2.7.0 from source code and install camellia-2.5.10 gem follow the guide here. Luckily, they can work together.
HornetsEyes require a lot of libs in order to build (Qt4, Boost, ImageMagick...). I failed to install Boost then I stopped.
When compared example code, I found Camellia is more friendly and easy to use. Let see
Camellia examples and one HornetsEyes example. HornetsEyes follows Class Oriented style (often found in C++/Java) in Ruby cloth.
I voted for Camellia: Smaller and Easier to use