Many of the rare earth metals are available in a distinctive form called dendritic (which means "branching"). When highly purified, these metals can be melted and when they begin to solidify do so in these wiry patterns. It is, in the commercial sense, an easy way to visually confirm a high level of purity as impurities otherwise disrupt the formation of dendrites.

We would also say that, visually, these are vastly more interesting to look at than the regular “lump” forms more commonly available.