Some mechanic (maybe involving forges?) that consumes valuable gems to add enchantment to weapons or rings, because gems tend to become useless at a fairly early point in the game.
The room of gems in the Castle should never contain worthless glass. (Unspecified whether it should still contain gray stones, or exclusively valuable gems.)
A new race whose primary feature is that it eats minerals: gets nutrition mainly from rocks, may be able to eat metals, and gains limited spell-like abilities based on the last precious gem they consumed. A couple non-comprehensive examples:
The abilities could be triggered using #monster, and would probably have a finite pool of uses, after which they can no longer be used. Eating another gem before this pool runs out would swap in new abilities and either give a fresh pool of uses or add to the existing one.
Several new objects for aiding in alchemy:
New objects “scrolls of riches”, which serve as a means of storing large amounts of money without carrying around all the literal gold.
Artifact whose main purpose is to soak up water and apply it to other things. It removes a pool or moat square or dries up a fountain when applied to one (moats may refill), gaining a charge each time. You can apply it to spend a charge blanking a scroll, erasing an engraving (any type, not just dust), cleaning your face or hands, and extinguish fiery monsters such as a flaming sphere (possibly instakilling them).
The initial idea was to create a “sponge” object as the base item type, but since it’s undesirable to make a new object just to be the base of an artifact, it could instead either be a “spongestone” gem, or just an artifact towel, since towels can already soak up water. (If implemented as a towel, the towel would only gain one charge of wetness when immersed in water, and could attempt to dry up that square immediately.)
When you throw a gem at a unicorn, it may vanish entirely, like a nurse does, instead of teleporting somewhere else on the level. This makes it unreliable to farm luck from them indefinitely and still have them around to kill later to get the horn and all the gems back.
The chance of vanishing could even be based on the number of gems it is already carrying, so that it won’t or is unlikely to vanish after throwing it a couple of gems but gets more likely after giving it lots of gems.
Track the amount of Luck increase you have had from a unicorn. If you steal gems from such a unicorn or kill a unicorn you have gotten Luck from, immediately revert that Luck increase.
Magpies, a bird that has a stealing attack; unlike monkeys, the stealing attack is limited to shiny objects (possibly based on their material) that are under a certain weight. Under that rule, rings, gems, and amulets are common targets of theirs. They probably have a rather fast speed, too. They should likely generate in small groups.
Magpies would have an alignment of 0 and lack the M2_HOSTILE flag, and thus may be peaceful to neutrals, but angering one makes all other magpies angry as well.
A magical bag which only holds gold and gems, but any gold and gems inside it don’t weigh anything.
Some mechanic (possibly a ritual spell) that enchants gems by disenchanting other items. Once enchanted, a gem can be used a single-use Pw reserve, magical grenades, or (rarely and lossily) tranfer their enchantment into some other piece of gear. Gem enchantment rules follow ring enchantment rules with respect to exploding when they get too full of magic.
Gnomes get permanent detection of gold and gems within a radius 3. Or possibly the radius can scale with XL.
Mines’ End variant that is an actual mine, with many twisty corridors dug out of the rock, and many unmined gems in the rock. The luckstone is unmined, in one of several possible spaces, not adjacent to any corridors but not too far away from them, and certainly not as far away as in the Wine Cellar. A pickaxe will be guaranteed somewhere on this level.
The Mines have a higher rate of gold and gems embedded in rock than the other branches.
Gold nuggets as a replacement for loadstones. They are expensive, but heavy, and generate cursed. Like loadstones, cannot be dropped when cursed.
Gloves of fortune, which unlike SporkHack cause killed monsters to drop d10 gold pieces that didn’t previously exist, and rarely a gem.
Dowsing rod: chargeable magical tool that when applied tells you the total value of all gold and gems on the level that isn’t being carried by the player (but not their locations), and whether there are any artifacts on the level that are not carried by the player.
Nuggets of various metals (copper, gold, iron, silver, mithril, maybe generic metal). These are all * class and are basically rocks but worth more.
Rock moles and other item-eating monsters drop any gold or gems they have eaten (digesting them can’t be a fast process), or they destroy 10% of all gold they eat and have a 10% chance of destroying any other items they eat; otherwise those items will be placed in the mole’s inventory.
Dwarf and gnome kings will never pick up worthless glass or gray stones on the ground (they continue to pick up all valuable gems).
Gnomes generated outside the Mines generate carrying one or two random gems (possibly none, subject to current level difficulty).
Unicorns (possibly only coaligned) can be tamed by throwing them a specific gemstone, which is deterministic for each individual unicorn and is random. Or else keep it simple and make it a low-ish chance per gem thrown.
Archeologists (maybe gnomes too) start the game with all valuable and worthless gems (but not gray stones) identified. If this is too powerful, perhaps they only begin the game with all worthless glass identified.
If you’re polyselfed to a lithivore, worthless glass tastes terrible but valuable gems taste delicious. Each gray stone gives a different YAFM: “Yabba-dabba delicious” for flint, “That was a heavy meal” for loadstone, “The stone’s texture is delightful/distasteful” for touchstone, ??? for luckstone.
More marginal corner-case uses for gems (similar to the amethyst/booze interaction):
