From Official Runecraft Wiki
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Ink Blocks is a technical term used in runeweaving to denote the basic material from which a rune is crafted.

Ink blocks are those of Tier 2 or higher. Various runes have a differing effect based on the Tier of their Ink.
All Ink in a given rune must be the same type as the other Ink blocks in that rune.
For example, if you were to make a Levitation Obelisk out of Bookshelves, all five of the blocks would need to be bookshelf.