When a gemstone is flawless, transparent, then light can refract and reflect well from the gemstone into the body which will result in better/powerful positive energy for the wearer and its surroundings. Natural, untreaded, gemstones are recommended because nowadays heat and diffusion treaments on gemstones like sapphire can even convert colourless sapphires to red sapphires (ruby) i.e. completeing changing astrological prospect of a gemstone. So, it is better to wear natural flawless gemstone for maximum benefits.
However, when someone doesn’t want to buy expensive gemstone then they can opt for substitute gemstones. For example in case of ruby one can go for Red Spinel, Red Garnet (i.e. Pyrope Garnet) gemstone.