Gems are a physical representation of magic stores, and are possibly magic's physical form in this world. It is not clear if all gems in Dominions are this way, or just a rare few; in-game events alternate between specifying that they're magical and not doing that. The world of Dominions is large and unwieldy, and discrepancies in text are to be expected for something that has been continuously updated for decades, but gems have always been the "currency" for expensive and impactful magic spells.
Nine paths of magic have a gem to them.
Gems are stores of magic, so they are most commonly found where ambient magic congregates in Magic Sites. Every nation has at least one, and new ones can be located in other provinces through Site-searching. There are Sites of every path, including Holy Magic Sites such as the Ancient Temple. Magic Sites that provide magic gems do so every turn.
Random events are another source of gems, albeit one less reliable. One of the significant draws for Fortune/Luck Scales (or even Misfortune scales) are the events that provide gems. There are also some events that take away gems, but these are significantly less common.
Note that, to collect gem income from Magic Sites into your treasury, there must be a Laboratory that can handle them; if not in the province, then at least in the same connected group of provinces. Commanders can carry gems (up to 100) and must do so to use them in combat, but gem income does not pour directly into their inventories if no lab is available. This treasury access works both ways; while commanders may trade gems between each other directly, they must be at a lab to withdraw from the treasury.
If you require a certain kind of gem (or at least more of it), you may wish to partake in Gem Alchemy (or simply "Alchemy") at a lab. This lets you convert two of any one gem type (except for Blood Slaves) into 1, or 2 into one of any gem (other than a Blood Slave), as many times as you want in a single turn. It is indeed inefficient. A few nations have secrets for perfectly magic-efficient alchemy, at least between specific gem types, but these processes take an entire month and are performed in small quantities per-mage.
For Blood Slaves, the primary source is a practice called Blood Hunting. This is a morally dubious practice of, rather than buying slaves from the Damned Merchant, yoinking virgins from the villages. It's certainly not always successful:
Failure at any stage results in 0 to 5 Unrest being generated. Success generates Unrest, too, anywhere from 1 to (three times the number of Slaves "produced") plus 4. That's the roll, d(slaves x 3 + 4). At least it's not open-ended. The default Slave yield in the Middle Ages is 1d6+(1d2-1), plus the hunter's blood level; or, in other words, 1-7 + .
Here's a table for what you can roughly expect in Blood Slave yields, in the Middle Ages, averaging the successes and the failures together. Note that these assume that you wait for Unrest to get back to zero before each attempt, which isn't ideal.
Blood Level | 5000 Pop Yields (0 Unr) | 7500 Pop Yields (0 Unr) |
---|---|---|
0 | 0.27 | 0.4 |
1 | 1.33 | 2 |
2 | 2.8 | 4.2 |
3 | 4.67 | 7 |
4 | 5.33 | 8 |
Entirely new to Dominions 6, the locals may Assassinate blood hunters, though the likelihood of this happening is reduced by friendly Dominion.
In the Early Ages, magic is abundant (perhaps too abundant) by default; Magic Sites are 18.2% more common than in the Middle Ages, and an additional 2d2-2 Blood Slaves (0-2) can be found while Blood Hunting.
In the Late Ages, magic is waning by default. Magic Sites are 18.2% less common than in the Middle Ages, and the Blood Hunting yield formula is modified to 1d6 minus (1d2-1); or, in other words, 0-6 + .
From casting Rituals to forging Items, to bearing the cost of high-Fatigue combat Magic, to even Empowering commanders, gems have a ton of uses.
For Combat Magic, a series of rules governs how mages can use gems:
For example, say a 4 mage needed to cast Relief, a spell that requires 5 and 1. He can spend 2 to cast the spell. This will inflict upon him the standard 100 Fatigue (ignoring Spellcasting Encumbrance), but he could spend 3 instead of 2 to reduce the burden to 50 Fatigue; or he could spend 4, to reduce the burden to 33. Since 4 is his skill level before spending gems in battle, 4 is his gem limit for a single cast.
Note that the gem cap per-turn can be raised by other "path-boosts", such as Communions, certain items, or spells such as Power of the Spheres.
On another note, Blood Slaves are alive. They have a unit type, the Blood Slave. They are abstracted in-game as rather pathetic units, and they can be killed in battle before you can sacrifice them. The Pain Transfer spell weaponizes this, in fact.
All Rituals must take place at a lab anyway, so the rituals draw straight from the treasury.