diff --git a/src/__tests__/materials.test.ts b/src/__tests__/materials.test.ts
new file mode 100644
index 0000000..383b04e
--- /dev/null
+++ b/src/__tests__/materials.test.ts
@@ -0,0 +1,38 @@
+import { describe, expect, test } from "bun:test";
+import {
+    BEHAVIOR_GAS,
+    BEHAVIOR_LIQUID,
+    BEHAVIOR_POWDER,
+    BEHAVIOR_SOLID,
+    type Material,
+} from "../materials/types";
+
+describe("material types", () => {
+    test("behavior type constants are distinct integers 0-3", () => {
+        const behaviors = [
+            BEHAVIOR_POWDER,
+            BEHAVIOR_LIQUID,
+            BEHAVIOR_GAS,
+            BEHAVIOR_SOLID,
+        ];
+        expect(new Set(behaviors).size).toBe(4);
+        for (const b of behaviors) {
+            expect(b).toBeGreaterThanOrEqual(0);
+            expect(b).toBeLessThanOrEqual(3);
+        }
+    });
+
+    test("Material interface has required fields", () => {
+        const sand: Material = {
+            id: 1,
+            name: "sand",
+            behavior: BEHAVIOR_POWDER,
+            density: 1.5,
+            color: [0.76, 0.7, 0.5, 1.0],
+            hardness: 0.1,
+            angleOfRepose: 34,
+        };
+        expect(sand.behavior).toBe(BEHAVIOR_POWDER);
+        expect(sand.density).toBe(1.5);
+    });
+});
diff --git a/src/materials/types.ts b/src/materials/types.ts
new file mode 100644
index 0000000..f26413a
--- /dev/null
+++ b/src/materials/types.ts
@@ -0,0 +1,23 @@
+export const BEHAVIOR_POWDER = 0;
+export const BEHAVIOR_LIQUID = 1;
+export const BEHAVIOR_GAS = 2;
+export const BEHAVIOR_SOLID = 3;
+
+export type BehaviorType =
+    | typeof BEHAVIOR_POWDER
+    | typeof BEHAVIOR_LIQUID
+    | typeof BEHAVIOR_GAS
+    | typeof BEHAVIOR_SOLID;
+
+// RGBA color, each component 0-1
+export type Color4 = [number, number, number, number];
+
+export interface Material {
+    id: number; // 0-255, index into lookup texture
+    name: string;
+    behavior: BehaviorType;
+    density: number; // relative weight, determines displacement order
+    color: Color4;
+    hardness: number; // 0-1, resistance to ant digging / degradation
+    angleOfRepose: number; // degrees, used in tier 2 gravity
+}