Some interesting replies, thanks
I just wanted to add a few ideas that may set some off researching more
The funny thing is... ALL gemstones ARE crystals lol! There is no difference when looking from a crystal healers point of view, though Coral, Pearl, and Amber are organics, they do not have a crystalline structure but are worn and set in jewellery and so commonly fall under the heading crystals or gemstones.
Each stone has its own unique properties and energy vibrations.
However from the ancient vedic gemologist's point of view ( and not all are in agreement with this) there are only 9 gemstones or nav-ratnas that count or matter and four more minor ones. All the others are thought to be inauspicious, bad luck or 'dirty'. Substitutions are thought to be dangerous.
Sun Ruby (Maanik) 2 Carats Gold, on ring finger
Moon Pearl (Moti) 2 Carats Silver, on ring finger
Mars Red Coral (Munga) 3 Carats Gold or Silver, on ring finger
Mercury Emerald (Panna) 1.5 carats Gold, on ring or little finger
Jupiter Yellow Sapphire (Pukhraj) 2 carat Gold or silver on middle finger
Saturn Blue Sapphire (Neelam) 2 carats Gold, on middle finger
Rahu Hessonite ( cinnamon garnet) (Gomed) 3 carats Gold or Silver, on middle finger
Ketu Cat’s eye Chrysoberyl (Lahsuniya) 3 carats Gold, in middle finger
.....only clean top quality gems are considered to be desirable. Sri Agni Puranam, chap. 246, slokas 7 & 8: states, "A gem free from all impurities and radiating its characteristic internal luster should be looked upon as an escort of good luck; A gem which is cracked, fissured, devoid of luster, or appearing rough or sandy, should not be used at all."
There are two widely debated opposing philosophies, viz., Anukul-vad v/s Pratikul-vad
ANUKULA-VAD: A Jyotish Gem will act "SAME" as it's planet - So gems should be chosen for Anukul (favorable) birth planets.
PRATIKUL-VAD: A Jyotish Gem will act "OPPOSITE" of it's planet - So gems should be chosen for Pratikul (unfavorable) birth planets.
The cost of these settings and gemstones is very expensive, much more than using a piece of raw ruby to balance a base chakra or holding a tumblestone of hematite for grounding. Perhaps thats why so few were able to practise it?