Assuming that this is a crossword or quiz or something, and so ignoring the more obscure high temperature and/or high pressure polymorphs (tridymite, cristobalite and the much more interesting coesite), the answer you are probably after is a crypto-crystalline polymorph known as chalcedony.
You may occasionally see this mineral cut into sections, polished, dyed lurid colours (commonly), and sold in tacky 'Gift Shops' under its more common name, agate.