I love new programming languages, and while I've been using Ruby for the past years, I've coded in Java, PHP, Crystal, Elixir and a host of others. I also love writing, and good documentation, which is hard to find. Crystal doesn't have a lot of that yet, so I'm pitching in!
I've written a blog post dedicated to this, but basically, Crystal is good at everything Ruby is not good at...plus as a Rubyist you already know some Crystal.
What am I getting?
Basically, you can read the whole thing online for free!
Do I have to know Ruby?
Really, I love alliteration: the only thing the 'for Rubyists' really means is that I assume you don't know about types, pointers, concurrency, or similar things. That's okay, you've never had to think about them before! I explain this stuff in extra depth. If you program in another dynamically typed language, you'll be just fine. If you program in another typed language, you'll be more than fine.