Star-shaped crystals form on stream ice when it gets very cold – in the single digits F. The smaller shapes are stars or crosses, but some forms are masses of pointed crystals. The snow at the edge of a stream can form dense mats of these crystals, and the icy surface can have clusters of starry crystals. Here are a few crystal islands on dark stream ice:
Frost crystals grow in different shapes, depending in part on the temperature. On one morning, I found the snowflake-like hexagonal plates you see in one of these images. All of the leaves around this one were covered with them. On another morning, I found rod-like frost crystals. When you get close enough, you can see that the rods are pipes of ice, hollow inside. Click an image to see it in a larger size:
This sort of image – frost on leaves scattered in the ground – is the sort of thing I take in late fall. In early January when I took these, the ground was still bare, and the leaf litter was still visible. Hard to imagine that now, with three or four feet of snow on the ground.