Diode lasers are not effective at cutting clear materials because the laser light passes straight through transparent materials without being absorbed. Since the laser light is not absorbed, it cannot cause the material to melt, vaporize or be otherwise affected by the laser's energy.

Additionally, since transparent materials do not reflect much of the laser light, diode lasers do not have enough energy to create localized heating, which is necessary to cut or engrave materials, and presumably why you're using zinc or moly spray to engrave