A gold alloy that doesn't behave like a gold alloy. Roughly 78% gold by mass, the rest aluminum, in a precise stoichiometric ratio that produces an intermetallic compound rather than a soft solid solution — and the result is a saturated violet-purple metal that breaks like glass under a hammer. Cannot be forged, cannot be soldered the way silversmiths solder, cannot even be drawn into wire. The way it gets used is the way a gemstone gets used: cast or sawn into shape, then bezel-set into a malleable yellow-gold or platinum carrier. Reads in the hand more like a hard ceramic than like a metal.
Stoichiometric AuAl2 — face-centered cubic Laves-phase intermetallic at ~78.6% Au / 21.4% Al by mass. The violet color is a band-structure optical effect: a sharp interband absorption edge near 2.2 eV produces high reflectance in the red-blue range and a deep absorption notch in the green-yellow, yielding visible violet rather than the Drude-mode reflection that gives elemental Au its yellow. Vickers hardness ~263 HV, Mohs ~7–8. Density ~11.6 g/cm³ (substantially less than 18k yellow gold's 15.6, because of the aluminum). Melting point ~1060 °C, congruent. Brittle — fracture toughness on the order of glass; cleaves on close-packed planes when struck. Cannot be hot-worked or cold-worked; standard goldsmithing solders attack the intermetallic and convert it back to a solid-solution Au-Al alloy with loss of color. Worked exclusively by investment casting from the melt, by lapidary methods (sawing, grinding, polishing as if it were a gemstone), and by light CNC milling with diamond tooling.
There is a true principle that the construction itself must vary with the material employed, and to that principle this purple gold submits its strange testimony. The hammer that draws the yellow alloy into a chalice would here only shatter the wall. The smith must learn that some matter wishes to be cast and never struck — that it speaks in fracture rather than in flow — and the design that respects this speech, setting the violet plate as one would set a stone, will read true; the design that ignores it, attempting to forge what cannot be forged, will read false in the hand of any honest workman.
Principled BSDF defaults derived from the sphere metallic finish. Reasonable seed for Blender, Substance, Keyshot, Rhino — tune per material. Or grab the whole library at once: ForMaterials library →
# finish: metallic albedo #4a3a78 metallic 1.00 roughness 0.25 ior 1.45 transmission 0.00 clearcoat 0.00 sheen 0.00 anisotropic 0.00
{
"albedo": "#4a3a78",
"metallic": 1.0,
"roughness": 0.25,
"ior": 1.45,
"transmission": 0.0,
"clearcoat": 0.0,
"sheen": 0.0,
"anisotropic": 0.0
}
# Blender 4.x — Principled BSDF
# Purple Gold (AuAl2) · finish: metallic
import bpy
mat = bpy.data.materials.new(name="mat_gold_purple_au_al2")
mat.use_nodes = True
bsdf = mat.node_tree.nodes["Principled BSDF"]
bsdf.inputs["Base Color"].default_value = (0.0685, 0.0423, 0.1878, 1.0)
bsdf.inputs["Metallic"].default_value = 1.000
bsdf.inputs["Roughness"].default_value = 0.250
bsdf.inputs["IOR"].default_value = 1.450
bsdf.inputs["Transmission Weight"].default_value = 0.000
bsdf.inputs["Coat Weight"].default_value = 0.000
bsdf.inputs["Sheen Weight"].default_value = 0.000
bsdf.inputs["Anisotropic"].default_value = 0.000
# KeyShot 11+ — lux Python API, Generic material
# Purple Gold (AuAl2) · finish: metallic
# Run from Window → Scripting Console
import lux
mat = lux.createMaterial(name="mat_gold_purple_au_al2", materialType="Generic")
mat.setProperty("diffuse", (74, 58, 120)) # 8-bit sRGB
mat.setProperty("metallic", 1.000)
mat.setProperty("roughness", 0.250)
mat.setProperty("indexOfRefraction", 1.450)
mat.setProperty("transparency", 0.000)
mat.setProperty("coatingWeight", 0.000)
{
"_format": "Substance Designer / Painter \u2014 pbrMetalRough constants",
"_about": "Purple Gold (AuAl2) \u00b7 finish: metallic",
"baseColor": {
"r": 0.0685,
"g": 0.0423,
"b": 0.1878
},
"metallic": 1.0,
"roughness": 0.25,
"ior": 1.45,
"opacity": 1.0,
"anisotropyLevel": 0.0,
"_notes": "Channels listed are the standard Substance pbrMetalRough output. Drop into a Uniform Color node per channel, or as the constant input on a layered stack."
}{
"asset": {
"version": "2.0",
"generator": "ForMatter"
},
"materials": [
{
"name": "mat_gold_purple_au_al2",
"pbrMetallicRoughness": {
"baseColorFactor": [
0.0685,
0.0423,
0.1878,
1.0
],
"metallicFactor": 1.0,
"roughnessFactor": 0.25
},
"extensions": {
"KHR_materials_ior": {
"ior": 1.45
}
}
}
]
}# USD Preview Surface — UsdShade.MaterialLook prim attributes
# Purple Gold (AuAl2) · finish: metallic
def Material "mat_gold_purple_au_al2" {
token outputs:surface.connect = </mat_gold_purple_au_al2/PreviewSurface.outputs:surface>
def Shader "PreviewSurface" {
uniform token info:id = "UsdPreviewSurface"
color3f inputs:diffuseColor = (0.0685, 0.0423, 0.1878)
float inputs:metallic = 1.000
float inputs:roughness = 0.250
float inputs:ior = 1.450
float inputs:opacity = 1.000
float inputs:clearcoat = 0.000
token outputs:surface
}
}
Editorial pass 2026-04-28 from Cretu & Van Der Lingen 1999 and Supansomboon et al. 2008.
House vocabulary — terms ForMatter uses with intent.
Materials and processes for people who design and make things.
A local-first library of materials, processes, applications, and finishes — equal weight, citable everywhere, with cost-over-volume curves, trade-off profiles, equipment-tier filters, and second-life paths layered onto the data so a student can move from "what is this" toward "what's actually buildable here, now, by me." Part of the renato.design ecosystem — sibling of Plenum, Specimen, Ingenue, gesture, graf, and the Renato Rhino plug-ins. Form and matter, inseparable.
Half of teaching materials is teaching how the material is made into the thing. The standard subscription library was always light on that half. The wedge here isn't better samples or a prettier interface — it's treating Process as a peer entity, not a footnote.
Conway's Material World on raw materials, Lefteri's Making It on processes, Forty's Concrete and Culture, Sparke's Design in Context, Bürdek's Design: History, Theory and Practice of Product Design, Schröpfer's Material Design on materials in architecture, Winchester's The Perfectionists on tolerance, Minshall's Your Life Is Manufactured on the global supply chain, von Busch's Making Trouble on material activism, Were's How Materials Matter, Hegger / Drexler / Zeumer's Basics Materials, Untracht and McCreight on metalsmithing, USDA Forest Products Lab on woods, GIA on gemstones, Schott / CoorsTek / Toray / Owens Corning datasheets, MakeItFrom for verifiable property numbers, ASM Handbook, ISO standards. Museum holdings draw from the Met, MAD, V&A, Smithsonian American Art Museum, Newark Museum of Art, British Museum, Heard Museum, Smithsonian NMAI, Eiteljorg Museum, Philadelphia Museum of Art, Cranbrook Art Museum, and Grand Rapids Art Museum — collection-record permalinks only, designer overview pages and exhibition listings excluded. Voice blocks now ride on every entry kind — material, process, application, and finish — and include Ruskin on iron, Anni Albers on twining, Greg Lynn on the shred-and-teeth NURBS lineage, Pugin on the metal that won't be hammered, Barthes / Yanagi / Benjamin channeled within their philosophy; Sparke, Bürdek, Forty, Conway, Schröpfer, Minshall, von Busch, Lefteri, Pat Pruitt, Mary Lee Hu, Tom Joyce, Albert Paley, and the rest of the contemporary makers quoted verbatim with citation. All cited.
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