How do snowflakes maintain symmetry?

Kat Arney put this question to physicist Stuart Higgins...

Stuart - Well, you need a big cloud of gas and water vapour and you need the right conditions - it needs to be nice and cold. And so, at the start of a snowflake, you need a speck, you need a nucleus, so typically this might be something like the dust in a cloud and the water will start to solidify and crystallise around that nucleus. And it just so happens that the way it packs is in a hexagonal structure - a regular hexagon has a symmetry of six and so that's what gives us this kind of growing six pronged structure of the snowflake.

Kat - And are all snowflakes different? Is this true or is this a myth?

Stuart - It is true to a certain extent, so they have similar structure. So snowflakes will share this common six pronged structure due to the fact that it's the packing of the water structure underneath. But what happens after that, when these arms spread out and they start to interlink and branch like tree branches, that's more dependent on the conditions, that could be the temperature and the exact pressure and all the different factors that might affect it. So, the chance of getting those two sets of conditions, to grow the same crystal in the same snowflake twice is very rare.