* Mineral Composition: Different minerals have different lustres. For example, quartz is typically glassy, while feldspars can be pearly or vitreous.
* Crystal Size: Fine-grained igneous rocks, like basalt, tend to have a duller, more matte luster, while coarse-grained rocks, like granite, can exhibit more distinct luster based on their constituent minerals.
* Surface Texture: The surface of an igneous rock can also affect its luster. A smooth, polished surface will reflect light differently than a rough, weathered surface.
Here are some common lustres found in igneous rocks:
* Vitreous: Glassy, like obsidian.
* Pearly: Like a pearl, often seen in feldspar.
* Metallic: Shiny, like metal, found in rocks with metallic minerals like pyrite.
* Dull: Not reflective, often seen in fine-grained rocks.
* Earthy: Like soil, often found in weathered rocks.
Therefore, when describing an igneous rock's luster, it's important to consider the specific mineral composition, crystal size, and surface texture.
