'Boule de Neige' means 'snowball' in French and that's a good description - dense mounds of globular little rosettes, so heavily covered in white hairs as to look like, well .... snowballs!
Like all Semps, the colour changes with the seasons - the rosettes start the year with a black/red flush but still densely cobwebbed. 'Boule de Neige' looks good all year but is at its best in summer; dry weather makes the balls tighter and more silvery white. Unlike some other densely hairy plants, it's a good grower even in wetter conditions.
Has become one of our favourites. Sometimes listed as 'Boule de Nigre' (Black Ball), presumably a confusion of the name and/or the spring colour.