Double snowflake. Snowflakes are ice crystals that form in air that has a temperature near the freezing point of water. If the air is calm, then a symmetrical, hexagonal snowflake can form. It is also quite common for two snowflakes to form from the same centre, due to a process called crystal twinning. This leads to a snowflake with 12 sides. These snowflakes have formed mostly by faceting, a crystal growth mechanism that forms straight edges and the hexagonal shapes of a plate snowflake. The arms of a snowflake form due to instabilities that cause branching growth from a snowflake's corners.