Gems in the Might and Magic universe are precious stones that are used for several different things throughout the games.
Might and Magic Edit
In Might and Magic, Gems are the second most important source of magic (the first one being Spell points). They are found throughout the world, usually locked in chests that can be found after winning a match. Different spells require different amounts of Gems, usually the stronger the spell is, the more Gems it requires.
A good way to find a lot of Gems is to search pirate caves. They usually have a few hundred gems in each for each person, making it enough for almost the whole game once you get a hold of a Pirate Map.
Might and Magic II Edit
In Might and Magic II, the situation is almost the same as in Might and Magic. However, there you can find a lot of Gems in a Gemmaker Volcano near Volcania, yet you are required to have the Levitate spell on.
Might and Magic VII Edit
In Might and Magic VII, spells rely entirely on spell points; gems can be found in the world as items and they have their names (Amethyst, Diamond, Ruby etc.). That way Gems are largely worthless and are only used as temporary gold containers - until the first town that has a Magic shop in it.
Heroes of Might and Magic Edit
In the Heroes of Might and Magic series, Gems are used as a resource. They are one of the rarer resources (as opposed to wood, ore and gold) and used mainly for building highest level creature dwellings and paying for those creatures. They are also commonly used for building Mage Guilds.
Gem mines, when flagged, with each produce one unit of gems per day.