Gem-quality corundum displaying any shade of red or pink is considered a ruby. In the United States, specific standards regarding saturation must be met for ruby to fall into that category; otherwise, it is labeled a pink sapphire. The grading of a ruby's color considers three aspects: hue, saturation, and tone. Hue refers to the ruby's color as seen, while saturation determines the visibility and intensity of the color. Tone tells us the purity of the color, indicating whether it has secondary hues or is a pure tone.