Tiers
From Official Runecraft Wiki
Most vanilla Minecraft blocks may be used in the construction of runes. Each block has been assigned a tier value based on their rarity and difficulty to obtain, which affects the potency of the rune.
Consult tier block for a partial list of tiered materials.
Note: Server admins may change which tier a material is as of runecraft 2.11. See disabled-runes.txt for details.