Luster is a descriptive term used in geology to describe the appearance of a mineral's surface in reflected light. There are many different types of luster, including:

* Metallic: Looks like a polished metal, such as pyrite.

* Non-metallic:

* Vitreous: Glassy, like quartz.

* Resinous: Like resin, such as amber.

* Pearly: Like a pearl, such as moonstone.

* Silky: Like silk, such as asbestos.

* Dull: No shine, like chalk.

* Earthy: Looks like dirt, such as hematite.

* Adamantine: Extremely brilliant, like diamond.

So, when you describe a rock as having a "metallic luster," you're saying that it reflects light in a way that makes it look like a polished metal.