Astrologers will suggest gemstones based on the 1 – 5 – 9 Lords these are yoga karaka planets in the birth kundali. When those are weak our mind & thinking will not work sharp normally. If we use gemstones based on the rays of the gemstones we think properly. It will improves our strength of soul and mind perfectly. The gemstone should not wear for 6, 8, 12 lord planets.
Some of western astrologers suggests the gemstones based on the 10th house. Most powerful Kendra place in the birth chart. If we wear the gemstone for that will work more efficiently for that planet.