Grey eyes are indeed very rare. They are most commonly found in people of Northern or Eastern European descent, particularly those with lighter skin tones. The rarity of grey eyes is due to the way light interacts with the iris and the amount of melanin in the eye. The grey appearance is often a result of the low melanin levels combined with the scattering of light in the stroma of the iris, creating a pale, almost silvery hue.
In terms of overall rarity, grey eyes are less common than blue or green eyes, making them a distinctive and intriguing trait. However, since they can appear similar to blue eyes under certain lighting, they may be overlooked.