一般CD上用作反射雷射光的是甚麼物料？

The reflective material coated on CD is typically aluminium.

Please refer to the web-page below:

https://en.wikipedia.org/wiki/Disc_rot

(Read the paragraph under "Causes")

2015-06-28 17:28:23 補充：
Your suppl question:
The reflectance of gold is only slightly better than aluminium at wavelength between 600 nm ~700 nm within the visible spectrum. If infra laser is used, reflectances of gold and aluminium do not differ much.

2015-06-28 17:30:24 補充：
(cont'd).....
Please refer to the first diagram on the web-page:

https://en.wikipedia.org/wiki/Reflectance

It is thus not too worthy for CD coated with gold.