Hm. Let me walk through my math. A water molecule weighs 0.00000000000000000000000002992 kg, and a snow crystal contains an average of 1,000,000,000,000,000,000 snow crystals. So, a snow crystal should weigh 0.00000002992 kg. The snowflake contains 100 snow crystals, so it weighs 0.000002992 kg. To convert that to grams, we multiply by 1000, so the snowflake weighs 0.002992 grams. A milligram is 0.001 grams, so a snowflake would weigh 2.992 milligrams on average, based on the figures I used.
This assumes, of course, that Mass of a Snowflake is correct, over on The Physics Factbook.

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