To easily install Crystal on Mac you can use Homebrew.
brew update brew install crystal-lang
If you're planning to contribute to the language itself you might find useful to install LLVM as well. So replace the last line with:
brew install crystal-lang --with-llvm
If you get an error like:
ld: library not found for -levent
you need to reinstall the command line tools and then select the default active toolchain:
$ xcode-select --install $ xcode-select --switch /Library/Developer/CommandLineTools
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https://crystal-lang.org/docs/installation/on_mac_osx_using_homebrew.html