Anna looks truly alarmed by their appearance, so it's safe to assume that this has never happened before. Because the diamond runes come from Elsa, they're most likely ice in nature, but they shine in various colors, adding to their mystique. What are they and why did Elsa generate them?
It's worth noting that these diamonds appear in the teaser poster for Frozen 2 as well. They connect to make a large snowflake, and just like in the trailer, they each have different designs. Are they sentient? Since it looks like Elsa didn't purposely create them, are they a sign of her losing control of her powers again or a manifestation of her growing stronger? We have a feeling it's a little of both.
And although the scene comes around after Elsa and Anna's conversation with Pabbie in the trailer, we can bet that the runes pop up in Arendelle before the sisters wind up in the forest and are part of the reason they're there in the first place.