I had a few issues getting the Crystal compiler to run on MacOS Sierra, these are the seteps I took to get it up and running.
Firstly install the Crystal Language through Homebrew;
brew install crystal-lang
Then Create and run a new Crystal app;
crystal init app my_app
cd my_app
crystal run src/my_app.cr
If like me you get some errors like ld: library not found for -lssl when trying to run the app, update and install xcode-select by running the following;
xcode-select --install
xcode-select --switch /Library/Developer/CommandLineTools
Then run your app again with crystal run src/my_app.cr and you should be up and running.