Not a gem expert but ............. just me!
Well, you've asked a good question and Sacha has actually answered it. It IS all about colour. However, what one person would call a Ruby, another may call it a Sapphire.
Here's an interesting article
http://www.ruby-sapphire.com/ruby_sapphire_borders.htm
For me, Rubies are primarily red with maybe some pink - but if the pink overtakes then it becomes a pink sapphire. You see, the thing is that sellers can charge more for Rubies so .............. you'll see more pink "Rubies" now than you would have years ago.
Below you'll see two rings I own. The marquise, for me, is a proper Ruby. The other ring, although certified as a Ruby, is too pink and I honestly would call it a Pink Sapphire - in some lights it looks red (1st photo) but in others it's a fuscia pink (2nd photo). It is
just on the cusp of being a Ruby.
You can buy natural, unheated, untreated Rubies but not from the shopping channels. Here:
http://www.africagems.com/unheatedruby.html Can you see how some are more pink and others definitely more red? Having said that, these are all what I'd call Rubies (unlike a certain channel as you say)!
http://www.naturalunheatedruby.com/ This guy is fairly expensive but specialises in natural untreated gemstones.
http://www.thenaturalsapphirecompany.com/Rubies/ Expensive but their CS is fantastic and the additional cost is worth it when you see the package that gemstones come in :tongue2:
Honestly? IMVHO there's too much sapphire on the jewellery channels being sold as Ruby.