Rubies are graded and valued based on several factors, including their colour, clarity, cut, and carat weight. The colour of a ruby is graded on a scale from light to dark, with the most valuable rubies being those with a deep, rich red colour. The clarity of a ruby is graded based on the presence or absence of inclusions, with clearer and more valuable rubies having fewer inclusions. The cut and carat weight of a ruby are also taken into consideration when determining its value.