You know this guy just found a turquoise.
While progressing my full read-through of the AD&D books, I rediscovered the "Reputed Magical Properties of Gems" table on DMG p. 26-27. It lists a bunch of gems and gem colors along with the things they're thought to aid, ward against, or otherwise be associated with. As per the paragraph following the table, the gems don't actually have these powers alone, or even when many similar gems are gathered. The table is intended only to provide some guidance for the DM:
"Regardless of what qualities gems, herbs, and other substances are purported to possess, the mere possession of a score of a type of gem or a bale of some herb will convey absolutely no benefit of magical nature to the character concerned. These special qualities are given herein merely as information for Dungeon Master use in devising special formulae for potions, inks, etc. The information might also prove useful in other ways, particularly with regard to description of magic items, laboratories, and so on. Under no circumstances should you allow some player to convince you to the contrary!" – DMG p. 27
The table is easy to skip over if you're glossing through the rules, or if you wave it off as horoscope-type nonsense without reading the attached paragraph. But it's got some gems in it.
Anyway, I'd like to draw your attention to the "Turquoise" entry, which reads, "Aids horses in all ways (but stone shatters when it operates)."
This is one of the more mysterious entries on the table. Runners-up include jet being "soul object material" and yellow gemstones being associated with jaundice. The turquoise entry is especially interesting because it's the only one that states that the gem is destroyed when it "operates," and the only one that explicitly says it applies "in all ways."
Let me tell you something: I love this shit. This is prime AD&D! Come for the gameplay, stay for the flavor, the absolute texture of Advanced Dungeons & Dragons. "Aids horses in all ways." What does it mean? I get what it means in context but what does it mean in the grand scheme? What line of thought put these hallowed words to paper? Incredible.
Lesser referees would pass this off as Gygaxian nonsense. But not you. You play by the book. Here's how you run turquoises at your table:
Magic items related to horses are often inlaid with turquoises. Unless they harm horses.
Potions and other magics which improve empathy with horses often include turquoise, since "all ways" includes mental wellbeing of the horse.
Spells which grant benefits to horses may want for turquoise (or the dust of that gem) as a spell component.
Possibly, magic which only aids horses in some ways might not include turquoise – or at least not a significant amount of the gem – since turquoise specifically aids horses in all ways.
Characters may be able to tell if a magic item which aids horses has ever been used, since shattered turquoise or entirely empty gem sockets could indicate that turquoise has shattered.
Horses may be desirous of turquoise if they have learned of its potential, especially in the case of intelligent or magic horses. Intelligent or magic horses may have turquoise in their lairs.
The possibilities are awesome to imagine, the implications staggering.