**Explanation:**

Lustre refers to the shininess or reflectivity of a material's surface. It is the property of a material to reflect light. Materials that have a smooth and polished surface tend to have a high lustre, while materials with a rough or dull surface have a low lustre.

Among the given options, the material that is not lustrous is **Wood**.

**Reasons why wood is not lustrous:**

1. **Surface Texture**: Wood has a porous and rough surface texture. The surface of wood is not smooth and polished, which prevents it from reflecting light efficiently. The uneven surface of wood scatters light in different directions, resulting in a dull appearance without much reflectivity.

2. **Absorption of Light**: Wood has a high capacity to absorb light instead of reflecting it. The composition of wood includes organic compounds, such as cellulose and lignin, which have high absorption properties. When light falls on the surface of wood, a significant portion of light is absorbed by the material, leading to a lack of lustre.

3. **Natural Variation**: The lustre of wood can also vary depending on the type and condition of the wood. Some types of wood may have a slightly smoother surface and exhibit a faint lustre, but it is generally much lower compared to materials like silver, gold, and diamond.

4. **Maintenance and Treatment**: Even if wood is polished and treated with varnishes or coatings, it does not achieve the same level of lustre as metals or gemstones. The natural properties and structure of wood limit its ability to reflect light and create a high shine.

In conclusion, wood is not considered lustrous due to its rough surface texture, high light absorption, natural variation, and limitations in achieving a high shine even with treatments.