A natural ruby typically has a deep, vivid red color. Look for a rich, vibrant red, often described as "pigeon's blood" red in the gem trade. The color should be evenly distributed, without any uneven "color spots".
Natural rubies are primarily red, but their hue can vary slightly with hints of other colors like pink, purple, or orange. The most prized rubies are those with a pure, vibrant red without these secondary hues.
Be cautious of rubies that have an unnaturally bright or fluorescent appearance, as this can be a sign of artificial coloring or a synthetic stone. High-quality natural rubies exhibit a strong, rich red saturation, without any brown or gray overtones. The more vivid and pure the red, the more valuable the ruby tends to be. Lower quality rubies might appear dull, with a more washed-out or brownish-red color - but that won't mean they are fake!
In a high-quality ruby, the color should be consistent and balanced throughout the stone. Uneven coloring, such as color zoning (areas of different colors) or patchiness, can reduce the stone's value.
Last but not least, check the ruby's response to light. Observe the ruby under different lighting conditions. Natural rubies should maintain their beauty and color under various light sources, from sunlight to artificial light. Due to their fluorescence, some rubies may appear more vibrant in daylight and slightly purplish under incandescent light.