Basically to restrict what can be built out of what there are a few tags for the metals: [WEAPON][WEAPON_RANGED][AMMO][DIGGER][ARMOR][ANVIL][ANY_USE]
Most of them are pretty self-explanatory for what they let the metal be used for (I believe any_use lets you build mechanisms and quivers out of it, but I'm not sure though). Armor includes shields here.
My idea was to restrict structural metals like durasteel to [DIGGER], [ARMOR] and [WEAPON_RANGED], which would allow players to build blasters and digging implements out of them. Adding a ranged attack and proficiency to the Gaffi sticks and vibroweapons (with the ammunition being impossible to aquire in-game) allows them to be classed as ranged weapons, like blasters. This means your durasteel/mandalorian steel/cortosis/phrik alloys can not be used to produce energy cells and lightsabers, but can be used for blasters, vibroweapons and normal melee weapons. An example for the slight "ranged" weapon hack is below:
[ITEM_AMMO:ITEM_AMMO_NULLAMMO]
[NAME:null:null]
[CLASS:NULLAMMO]
[DAMAGE:1:BURN]
[ITEM_WEAPON:ITEM_WEAPON_AXE_VIBROAXE]
[NAME:vibroaxe:vibroaxes]
[DAMAGE:150:SLASH]
[SKILL:AXE]
[RANGED:AXE:NULLAMMO]
[WEIGHT:50]
[TWO_HANDED:7]
[MINIMUM_SIZE:6]
[MATERIAL_SIZE:4]
[ITEM_WEAPON:ITEM_WEAPON_GADERFFII]
[NAME:gaderffi:gaderffi]
[DAMAGE:100:BLUDGEON]
[SKILL:MACE]
[RANGED:MACE:NULLAMMO]
[WEIGHT:50]
[TWO_HANDED:6]
[MINIMUM_SIZE:5]
[MATERIAL_SIZE:3]
By creating a set of "metals" for the lightsaber crystals that can only be used for [WEAPON]s, you will end up with crystals that can only be used for one thing, building lightsabers. They can be made suitably rare and of different qualities to fit your vision of the mod.
Another set of "metals" could be created to form energy cells of differing qualities that only have the [AMMO] tag, possibly only being available through reactions (basically it's like creating a new alloy). This means rather than seeing "durasteel blaster bolts x25" you instead get something like "Arkanian resonance cell blaster bolts x25" or whatever name you choose.
[Anvil] tags can be added to metals depending on whether you think anvils should be a large, industrial fixture or a precise workbench (structural metals if large fixture, lightsaber crystals/energy cells if you want a more high-tech feel).