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This is a paid asset, but now you can download HQ PBR Rock Materials Free.
HQ PBR Rock Materials v1.0
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This package contains 8 high quality materials based on 2048×2048 textures made out of rock photos. Each materials have albedo, specular, normal, displacement (height) and occlusion maps. Each texture is perfectly tileable. Rocks where photographed in a western Europe mountain forest.