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.

• Fire uses red Fire Gems, sometimes called rubies.
• Air uses white Air Gems, sometimes called diamonds.
• Water uses blue Water Gems, sometimes called sapphires.
• Earth uses orange Earth Gems, with a liquid form known as "earth blood".
• Astral uses clear Astral Pearls.
• Death uses purple Death Gems.
• Nature uses green Nature Gems.
• Glamour uses prismatic Glamour Gems, sometimes called crystals.
• Blood uses Blood Slaves.
• Holy has no gems.

Gems are stores of magic, so they are most commonly found where ambient magic congregates in magic sites. Every nation has at least one in its capital, and new ones can be located in other provinces through site searching with mages. There are sites of every path, including Holy 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.

For gems to be collected from a province's magic sites into your treasury, the magic site must be connected by friendly provinces to a laboratory. Commanders can carry gems (up to 100) and must do so to use them in combat, but can only be given gems by another commander, or directly from the treasury at a lab.

Gems can be converted into other types via alchemy by any mage at a lab. Two gems of the same type can be alchemised into one astral pearl, while two astral pearls can be alchemised into any other gem type. Blood slaves cannot be alchemised. The pearl cultivator ability allows alchemisation of specifically water gems to astral pearls at a 1:1 exchange rate, but only a limited amount per commander per turn.

Although there are a few magic sites that grant blood slaves, the primary method of gathering blood slaves is blood hunting. See the main article for more details.

In the Early Ages, magic is 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 plethora of uses.

Gems can be used in combat to cast spells that require gem expenditure, and also to reduce fatigue generated by spells. See the main article for more details.

Unlike other gem types, blood slaves are not just contained in a commander's inventory but also represented by actual units in combat. They are fragile and can be killed before the commander uses them.

All rituals must take place at a lab anyway, so the rituals draw straight from the treasury.