Forbidden and Arcanus: Getting Started with Arcane Crystal
Forbidden and Arcanus adds vanilla-styled magical content, and its progression begins with Arcane Crystal and a special furnace. This guide covers the early materials.
Arcane Crystal
Arcane Crystal is the mod's core magical material. Smelt or blast 
Arcane Crystal Ore into a crystal, and smelt a crystal further into 
Arcane Crystal Dust. The dust appears throughout the mod — for example, four 
Bone Meal with Arcane Crystal Dust makes 
Arcane Bone Meal, and dust builds the 
Aureal Bottle.
Edelwood and Deorum
Edelwood is the mod's dark tree; smelting an 
Edelwood Log yields 
Dark Matter, and Edelwood planks build a themed set. Deorum is a golden magical metal used to upgrade blocks — combining 
Edelwood Planks with a 
Deorum Ingot makes 
Arcane Edelwood Planks, and the same trick upgrades Polished Darkstone.
The Clibano
The centerpiece early machine is the Clibano, an upgraded combustion furnace that smelts faster and unlocks recipes an ordinary furnace cannot — producing Arcane Crystal, ingots, 
Obsidiansteel Ingot and 
Rune. Building a Clibano is the gateway to the mod's materials, covered in the Clibano guide.
The older build has more content
This is unusual enough to lead with. 1.20.1 carries 172 crafting recipes against 144 on 1.21.1, 39 stonecutting recipes against 14, and a Combine Residues system with 10 recipes that does not exist on 1.21.1 at all.
The newer build adds one smithing transform and one modifier recipe. Otherwise the 1.20.1 line is the fuller one, which reverses the usual expectation.
What is identical
Clibano Combustion at 15 recipes and smelting at 6 are the same on both. The clibano is the mod's signature furnace, so its stability is convenient.
No structures
The mod places no structures, so nothing here is found by exploring — 120 items and 108 blocks all come from crafting and processing.
Where to start
Build the clibano. With 15 recipes on either version and no version-specific behaviour, it is the one part of the mod any guide will describe accurately for your build.
Planning around the version gap
Because 1.20.1 carries more recipes overall, a player moving from that build to 1.21.1 will find things missing rather than added — the residue system in particular, and roughly two thirds of the stonecutting list. Check a recipe viewer before planning a build around a decorative block you remember.
Polished Darkstone