What is the name of a very hard gem that looks like clear glass?

Homework Help!What is the name of a very hard gem that looks like clear glass?
Quartz.What is the name of a very hard gem that looks like clear glass?
There are so many it is difficult to compile a list! But if I ignore the responses I see already, I would say zircon (not cubic zirconia, zircon is a natural stone mined in Cambodia), YAG (short for yttrium aluminum garnet- grown in the lab), colorless sapphire (also called corundum), GGG (another lab stone), topaz (sometimes colorless but not always), synthetic spinel and natural spinel, synthetic moissanite, synthetic rutile, synthetic colorless sapphire, and plain old man-made glass is also used. How do you define ';hard';? Corundum has a hardness of 9 on the mohs scale, and diamond is a 10. The best choice (other than diamond) is colorless sapphire, also called colorless corundum. It occurs in nature, but can also be made in the laboratory. Maybe your teacher wants the answer ';diamond';.
Diamond or Cubic Zirconia, although diamond is one of the hardest substances known to us.
quartz