Taking this in another direction, because the OP rejected my first answer,
Shiny as a diamond
Rubies are shiny, as are diamonds.
Once thought to be under the sea
the two proposed names for the language were Ruby and Coral.
Better than a snake
Ruby advocates claim it is better than Python (citation needed)
I seek to bring joy to many
Matsumoto, Ruby's creator wrote "For me the purpose of life is partly to have joy. Programmers often feel joy when they can concentrate on the creative side of programming, So Ruby is designed to make programmers happy."
You are my master, I control your slave
Programmers control computers
Read my words only where defined
A reference to undefined variables
If you don't exist, to me, you're a liar
Undefined values evaluate as false in Ruby, as in many languages.
Tell me what I am, if you'd be so kind.
The Ruby language