Hi Amit,
Yes, you can use gemstones for controlling ailments. However, they are like homeopathy; they will give results after some time (but not immediate). I advice you to continue taking your regular medicine for blood pressure; and along with that, you can wear the gemstones for astrological remedies.
In your grandma’s case, the green colored stone (most propabaly Emerald) is significator of planet Mercury. Usually mercury controls our blood circulatory system like veins and nerves. It will give strength to veins and nerves which indirectly avoids heart related issues and diseases such as brain hemorrhage.
If you really want to control high blood pressure, you should use Neelam (Blue Sapphire) as it’s dead against enemy of Mars (significator of blood). By using this stone, the blood pressure starts getting under control.
Beware, NOT to use this stone for low blood pressure patients as this will risk the person by further lowering the blood pressure. Hence; for people with low blood pressure, Coral should be used.
All these remedies shouldn’t be used without consulting an eminent astrologer (with proper study of your horoscope). We need to study your natal chart whether the above said general stone are yet favourable or unfavourable.
GOD BLESS!!!
Srinivasa Temkar