If you install the “sqlite3-ruby” gem and it prints something like this:

Building native extensions.  This could take a while...
Successfully installed sqlite3-ruby-1.2.2
1 gem installed
Installing ri documentation for sqlite3-ruby-1.2.2...
Installing RDoc documentation for sqlite3-ruby-1.2.2...

And when you try to use it in your rails application you get an error like this:

rake aborted!
no such file to load -- sqlite3/database

(See full trace by running task with --trace)

You’re probably missing the libsqlite3-ruby package.

sudo apt-get install libsqlite3-ruby