Snow - YES! Snow crystallizes in the hexagonal crystal system (six-sided crystals called "snowflakes"?)
Galena - YES!
Ice Cubes -- NO! They are made by a machine so they are not natural.
Graphite - YES, if it is found in nature. NO, if it is made in a machine or laboratory.
Ruby - YES, if it is found in nature. NO, if it is made in a laboratory, where much ruby comes from.
Cubic Zirconia - NO! It is made by humans in a laboratory.
Diamond - YES!
Fluorite - YES!
Opal - NO. Opal does not crystallize in a crystal system. It is called a "mineraloid" which means it is very much like a mineral, but does not meet all of the 5 criteria for officially being called a mineral.
