The premise is that there are 12 Crown Jewels. Like Knights, they form a single pile. In a 2 player game, 8 are chosen at random to play with. Each one is a Treasure-Victory-Gem (Gem is a new type, like Knight), which functions as a Copper but is worth a Duchy if a certain condition is met. The condition varies from gem to gem. Each has a cost of $4, but can be gained for free when you purchase a non-Victory costing at least $5 so that you have an incentive to gain the gem early on before you know for sure whether you'll be able to meet the condition. In other words:
Basic Template
Gemstone
$4 Treasure-Victory-Gem
$1
Worth 3 VP if... [condition]
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In games using this, when you buy a non-Victory costing at least $5, you main gain a Gem.
Here are my ideas, based on this
list of birthstones. Generally I erred towards making the conditions easy to meet, since you don't want to be diverting too much attention towards just a few VP. I'm aware that Bloodstone is impossible to activate on some boards, but it is doable on many, and may be worth gaining as Trash-for-Benefit fodder in others.
Garnet$4 Treasure-Victory-Gem
$1
Worth 3 VP if you have at least 30 cards in your deck.
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In games using this, when you buy a non-Victory costing at least $5, you main gain a Gem.
Amethyst$4 Treasure-Victory-Gem
$1
Worth 3 VP if you have at most 30 cards in your deck.
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In games using this, when you buy a non-Victory costing at least $5, you main gain a Gem.
Bloodstone$4 Treasure-Victory-Gem
$1
Worth 3 VP if you have at least two Attack cards in your deck.
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In games using this, when you buy a non-Victory costing at least $5, you main gain a Gem.
Diamond$4 Treasure-Victory-Gem
$1
Worth 3 VP if you have at least three Action cards costing $5 or more in your deck.
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In games using this, when you buy a non-Victory costing at least $5, you main gain a Gem.
Emerald$4 Treasure-Victory-Gem
$1
Worth 3 VP if you have an Estate, Duchy, and Province in your deck.
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In games using this, when you buy a non-Victory costing at least $5, you main gain a Gem.
Moonstone$4 Treasure-Victory-Gem
$1
Worth 3 VP if you have at least one Curse in your deck.
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In games using this, when you buy a non-Victory costing at least $5, you main gain a Gem.
Ruby$4 Treasure-Victory-Gem
$1
Worth 3 VP if you have at least 5 Action cards in your deck.
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In games using this, when you buy a non-Victory costing at least $5, you main gain a Gem.
Peridot$4 Treasure-Victory-Gem
$1
Worth 3 VP if you have no Duchies in your deck.
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In games using this, when you buy a non-Victory costing at least $5, you main gain a Gem.
Sapphire$4 Treasure-Victory-Gem
$1
Worth 3 VP if you at most two Attack cards in your deck.
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In games using this, when you buy a non-Victory costing at least $5, you main gain a Gem.
Opal$4 Treasure-Victory-Gem
$1
Worth 3 VP if you have at most 5 Action cards in your deck.
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In games using this, when you buy a non-Victory costing at least $5, you main gain a Gem.
Topaz$4 Treasure-Victory-Gem
$1
Worth 3 VP if you have at least 3 Gems in your deck.
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In games using this, when you buy a non-Victory costing at least $5, you main gain a Gem.
Turquoise$4 Treasure-Victory-Gem
$1
Worth 3 VP if you have at most 3 Gems in your deck.
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In games using this, when you buy a non-Victory costing at least $5, you main gain a Gem.