The variants you find in CoreShards.xml are hardcoded, as are the global shard power bonuses.

There should be no problem adding "faux" shards as world resources though. A custom shard power wouldn't be able to boost spells directly, but there is a work-around if you are interested in a bit of tweaking.

Yes, but the xml is not going to work exactly the same as normal shard powers because those are hardcoded. The functionality will be the same in-game.

First you need a continuous resource. A continuous resource functions like food did in E:wom - world resources produce X each turn, no stockpile is gathered. If you have one, it will say that you have 1 all the time.

   <ResourceType InternalName="rivermod_magmatype_spellres">
        <DisplayName>Magma Forge</DisplayName>
        <Type>MagmaForge</Type>
        <IconColor>0,0,0</IconColor>
        <Icon>Stat_AttackFire_Icon.png</Icon>
        <ClothIcon>gfx\\TacticalIcons\\MetalOre_1.png</ClothIcon>
        <ModelColor>0,0,0</ModelColor>
        <Worth>5</Worth>
        <Global>1</Global>
        <Rarity>0.0</Rarity>
        <Stored>0</Stored>
        <TradedByCaravans>0</TradedByCaravans>
        <ShownInGlobalDisplay>1</ShownInGlobalDisplay>
        <Medallions InternalName="Ration_Res_Medallions">
            <All>Res_Ores_Plains.png</All>
        </Medallions>
</ResourceType>

then you need a world resource

    <ResourceType InternalName="Resource_IronOre">
        <DisplayName>Iron Ore</DisplayName>
        <Type>IronOre</Type>
        <Description>Build a Mine here to produce Metal.</Description>
        <LongDescription>The ground here has been ripped asunder by the cataclysm; rubble is strewn everywhere, some of it rock shot through with veins of reddish and purple stone. This is iron ore in its raw form.  Build a mine here, and extract it to forge powerful weaopns and armor. </LongDescription>
        <BarredTerrain>River</BarredTerrain>
        <BarredTerrain>HillsTerrain</BarredTerrain>
        <PreferredTerrain>Category:Land</PreferredTerrain>
        <MinPerPlayer>3</MinPerPlayer>
        <Rarity>400</Rarity>
        <NeedsToBeUnlocked>0</NeedsToBeUnlocked>
        <HarvestType>Mining</HarvestType>
        <Production>1</Production>
        <Worth>5</Worth>
        <Global>0</Global>
        <Stored>0</Stored>
        <RummagedPerTurn>0.75</RummagedPerTurn>
        <OnSelectSFX>Click_Iron_Mine</OnSelectSFX>
        <GameModifier>
            <ModType>Resource</ModType>
            <Attribute>TileYieldMaterials</Attribute>
            <Value>5</Value>
            <PerTurn>1</PerTurn>
        </GameModifier>
        <AIData AIPersonality="AI_General">
            <AIPriority>3</AIPriority>
        </AIData>
        <Medallions InternalName="">
            <All>Res_Ores_Plains.png</All>
        </Medallions>
        <Icon>Resource_IronOre.png</Icon>
        <IconColor>0,0,0</IconColor>
        <ClothIcon>Resource_IronOre.png</ClothIcon>
        <TileDesign>R_Iron_Ore_01</TileDesign>
        <TileDesignForTerrain>DesertTerrain:R_Lombard_Iron_01</TileDesignForTerrain>
        <TileDesignForTerrain>Imperium:R_Iron_oar_Imperium_01</TileDesignForTerrain>
        <TileDesignForTerrain>Scrapyard:R_Scrapyard_Iron_01</TileDesignForTerrain>
        <ModelColor>0,0,0</ModelColor>
    </ResourceType>

then you need an improvement on the resource. Note below how the <SupportedResourceType>IronOre</SupportedResourceType> in the improvement matches the <Type>IronOre</Type> in the Resourcetype just above this one. Also note that the resource being created (<Attribute>MagmaForge</Attribute>) has to match the <Type>MagmaForge</Type> from the resource definition (the first Resourcetype at the beginning of the post).

    <ImprovementType InternalName="IronMine">
        <DisplayName>Iron Mine</DisplayName>
        <Description>Iron Mines can be built on Iron Ore to produce 1 Metal.</Description>
        <IgnoreDuplicateImpLimit>1</IgnoreDuplicateImpLimit>
        <RequiresResource>1</RequiresResource>
        <SupportedResourceType>IronOre</SupportedResourceType>
        <PreferredTerrain>Category:Land</PreferredTerrain>
        <GameModifier>
            <ModType>ConstructionResourceCost</ModType>
            <Attribute>Gold</Attribute>
            <Value>-10</Value>
        </GameModifier>
        <LaborToBuild>100</LaborToBuild>
        <GameModifier>
            <ModType>Resource</ModType>
            <Attribute>MagmaForge</Attribute>
            <Value>1</Value>
            <PerTurn>1</PerTurn>
            <Provides>+1 Metal</Provides>
        </GameModifier>
        <AIData AIPersonality="AI_General">
            <AIPriority>5</AIPriority>
            <AITag>Mining</AITag>
        </AIData>
        <ArtDef>Art_IronMine</ArtDef>
    </ImprovementType>

Now for the final trickery - we also need to be able to use nature shard powers like normal shard powers. That's not possible, so we need to make a work-around.

First, we create a new unitstat. It is going to be default to 1. That means everyone is always going to be walking around with 1 in this (unless you control nature shards - more on that after the unitstat).

  <UnitStatType InternalName="UnitStat_AvalonCalling">
    <DisplayName>ISMdfasasdfOUNTED</DisplayName>
    <DisplayNameShort>DGE</DisplayNameShort>
    <Description>The chance to dodge an attack in battle.</Description>
    <Icon>Stat_Dodge_Icon.png</Icon>
    <Hidden>1</Hidden>
        <UnitStatGrouping>CombatStat</UnitStatGrouping>
        <UnitStatGrouping>UnitDetailsCombatStat</UnitStatGrouping>
    <DefaultValue>1.0</DefaultValue>
  </UnitStatType>

Then, we need to create a way to globally buff this unitstat for all your controlled units. Enabling global buffing of unitstats requires its own xml. Note how the internalname mirrors the internalname of the UnitStatType but with an added "A_", essentially "A_*".

      <PlayerAbilityType InternalName="A_UnitStat_AvalonCalling">
        <DisplayName>Unit Hit Point Bonus</DisplayName>
        <Description>Provides a bonus to the hit points of all units.</Description>
    <Icon>Kingdom_Crest_02.png</Icon>
</PlayerAbilityType>

Now we go back to the improvement above and add the following:

      <GameModifier>
        <ModType>Player</ModType>
        <Attribute>AbilityBonus</Attribute>
        <StrVal>A_UnitStat_AvalonCalling</StrVal>
        <Value>100</Value>
      </GameModifier>

The end result is that all units start with 1 in your unitstat. But for each improvement controlled, that value is increased by 100%. If you control 2, it is increased by 200%. So a unit with 5 controlled improvements will have 6 in the unitstat. Then you can use that unitstat for all calculations in spells.

As you have probably figured out by now, the continuous resource we introduced above is actually not used for anything except to give a visual count of how many of that shard is controlled.

 PS. You'll need to change a bunch of stuff not to conflict with vanilla game (or my mods), obviously.

PS 2. Since the world resource is not technically a true shard, Corruption spell and Consume spell will not work on them.

I wrestled with this for many hours until I finally broke down and moved it from my mod directory to the game Data\English directory. Just a note for anyone else who attempts to do this in the future, this will not work in the mod directory!

Absolutely. For testing I set the spawn chance high, then once I was satisfied it worked correctly, I reduced the spawn chance to be the same as shards. 

They work exactly like regular shards with the only exception being that the indicator for the number of shards you control appears in the upper left instead of the upper right and I had to do the calculations on some spells slightly different.

Shards are hardcoded and any resources you make are treated like regular resources. I've thought about redoing the existing shards so that everything appears on one side of the screen. The only thing stopping me is a design decision.

I am going to use one of two schemes for magic:

Life

Death

Air

Earth

Water

Fire

Astral

Blood

Nature

(Basically like Dominions 3)

Or I am going to use this system:

Life

Death

Chaos

Sorcery

Nature

(A combination of "Magic the Gathering" and "Master of Magic".

If I go with the former, I don't know that I want to fool with changing the existing shards just for a very simple visual change, If I go with the latter, I will probably end up changing them. 

Well, I've done that with heavenfall's help. I made nature shards that function exactly like regular shards, with the UI difference. You cannot add shards that are 100% identical to the existing ones because it is hard coded at the moment. But you can mimic them so closely that the UI difference is the only issue. So with that being said, you could take the "hard route" and redo all the shards, eliminating the original ones, and making them look uniform in the UI.

There may be a way to modify the UI and do what you are asking, but that is beyond me. Even if you do it the way I am currently doing it, it is barely noticeable. I intend to make it look nice eventually, but I don't want to devote that much time to a visual change this early in modding when I have races, factions, classes, tech trees, spells, etc to worry about and limited time at the moment to mod.

This evening if I get a chance, I will post a screenshot of what it looks like doing it the way I am doing it now.

This is what it does with the shards, not really that big of a deal at all:

Not inherently, but there may be a way to modify the existing spell and give it an extra effect or create a new spell that does. I plan to throw the spell out anyway so not a biggy for me, but it may be important to people who plan to keep things pretty vanilla.

Tried but couldn't find an expression that returns anything, such gold or mana. I almost suspect they didn't make such an expression...

For mana you can specifically use <Expression><![CDATA[[UnitOwner_GetMana]]]></Expression> (see manablast). However, there is no way to access new resources dynamically.

hardcoded in the .exe