CNC Glass Engraving

Have been trying to glass engrave on my router with a cheapo chinese sprung tool. At first it cut OK but soon faded. I used both drag and rotational (@ 4000 rpm) but not sure if it was intended to be rotational.
Some questions then if I may. - Can anyone recommend a good but not stupidly expensive tool?
Is the glass to engraved a significant factor, i.e. I was trying to cut on toughened glass?
Seems simple enough to make the sprung piston drag type, but is drag the way to go as opposed to rotary?
Cutting tips, composite or solid diamond/tungsten?
If you are practiced in this application I would very much appreciate your kind assistance.