Colour moves the price of a ruby more than anything else. A strong, pure red with no brown or orange is the most valuable.
Clarity comes next. Some fine inclusions are normal in natural rubies and can even help confirm a stone is natural; what matters is whether they dull its brightness.
Famous origins can add a premium, but a well-coloured stone from a lesser-known source is often the better value. Always ask whether the ruby has been heated or treated.