A trademark or service mark can be anything that is distinctive of the source, including the luster of the the product.  Thus a trademark might be a metallic surface:

a pearlescent surface:

a matte surface:

a glossy finish:

a frosted surface:

a sparkly surface:

a shiny surface:

and a reflective surface:

Whether incorporated into a product for that purpose, or a happy accident of design, any feature of a product that customers can recognize as indicating source, can be a trademark.