9 Gemstones:
1) Ruby (माणिक्य) - Sun
2) Pearl (मोती ) - Moon
3) Red Coral (मूँगा) - Mars
4) Diamond (हीरा) - Venus
5) Emerald (पन्ना) - Mercury
6) Yellow Sapphire (पुखराज)- Jupiter
7) Blue Sapphire (नीलम) - Saturn
8) Hessonite (गोमेद) - Rahu
9) Cat's Eye (लहसुनिया) - Ketu
Astrologers/Schools follow different theories to choose Metals in which gemstones should be studded. Listen to your astrologer for it. However, I will tell 1 of such methods. Read 4/x too.
Metals for:
Sun/Jupiter = Gold (सोना)
Moon = Silver (चांदी)
Mars = Copper ( तांबा)
If Sun or Jupiter is good in chart, gemstone can be worn in Gold.
If Sun/Jupiter are not good, but Moon is good, gemstone can be worn in Silver.
If Sun/Jupiter/Moon are all not good in chart and Mars is good, Gemstone can be worn in Copper.
If all of Sun, Jupiter, Moon and Mars do not give good results in chart, then Gemstones can be studded in Panchdhatu.
Precious gemstones like Diamond, Blue Sapphire, Emerald etc. are studded only in Gold because of its strong gripping power so that gemstones are not lost.
Some people get studded Blue Sapphire/Diamond in Silver too as it has decent grip, and Silver does not become dull after sometime. But studding precious gemstones in Copper or Panchdhatu is risky as Gemstone can slip from loose grip, and Copper becomes dull in appearance.
So, even if Mars is good in chart, do not stud Diamond/Blue Sapphire/Emerald etc. in Copper. Get them studded in Gold/Silver only.
Prescribing metal depends on schools of Jyotish but nothing stops us from applying practical mind.
Pearl is not studded in Gold because if Moon is good in chart, silver automatically qualifies as a metal. Same with others.
Uparatnas/Substitutes do not have any astrological powers, so do not count them among gemstones. You can wear them, they do not harm.
Some schools prescribe gemstones for specific period, some for lifetime and some for specific purpose. (e.g. Marriage).
I believe wearing incorrect gemstone harms native, so better not wear if unsure. Some gemstones may help in one field but harm in another aspect.
Finally, I do not believe in testing effect of gemstones by keeping them under pillow or in pocket. Whether they would suit or not is evident from birth chart. Some astrologers recommend to test this way.
People not aware of their birth time should NEVER wear ANY gemstone
People who do not know their birth time or are unsure of correct birth time should NEVER wear any of 9 gemstones mentioned above even if it's cheap.
Weight:
Gemstone above 5 Rattis can be worn. Diamond should be of minimum 30 cents. Person's Weight is irrelevant.
Pearl - Monday
Coral/Cat's Eye- Tuesday
Emerald - Wednesday
Yellow Sapphire - Thursday
Diamond - Friday
Blue Sapphire/Hessonite - Saturday.
Wash with Milk+Gangajal, Face East side, chant Beej Mantra of Graha 108 times and wear it.
Ruby, Sun's gemstone is worn on Sunday.
To make quick money, some people recommend wearing gemstones of Trikon (1,5,9) houses. They also sometimes give gemstone of one house & substitutes of other two.
In any case, never wear it because ALL Lords of Trikon houses may not be giving good results.
If ring/maala of Trikon Lords contains original gemstones (which will be very costly of it has Blue Sapphire or Yellow Sapphire or Diamond), and if Lords of Trikon are not good in your chart, you will buy problems by paying from pocket.