@gailcalled That is a great link. I had not considered the effect of O16 vs O18 and Deuterium. I thought the difference was just due to the micro-climate where the crystal formed. In a lab it is possible to make 2 snowflakes that look alike, but to truly BE alike we need to fIgure all the isotopes of each molecule.
From the link above:
“If we restrict ourselves to water molecules which contain two ordinary hydrogen atoms and one ordinary 16O atom, then again physics tells us that all such water molecules are exactly alike. However about one molecule out of every 5000 naturally occurring water molecules will contain an atom of deuterium in place of one of the hydrogens, and about one in 500 will contain an atom of 18O instead of the more common 16O. These rogues are not exactly the same as their more common cousins.
Since a typical small snow crystal might contain 1018 water molecules, we see that about 1015 of these molecules will be different from the rest. These unusual molecules will be randomly scattered throughout the snow crystal, giving it a unique design. The probability that two snow crystals would have exactly the same layout of these molecules is very, very, very small. Even with 10^24 crystals per year, the odds of it happening within the lifetime of the Universe is indistinguishable from zero.”