When buying ruby gemstones, it's important to look for natural rubies that have a good color, clarity, and cut. The color of a ruby should be a rich, deep red and should not have any brown or purple hues. The clarity should be good, with minimal inclusions or blemishes. The cut should be precise and symmetrical to maximize the stone's brilliance. Additionally, it is recommended to look for certifications from reputable organizations such as GIA or AGL to ensure authenticity and value of the gemstone.