Perennial subshrub cultivar of the Asteraceae family, selected from Helichrysum amorginum, a species native to the Greek Cyclades islands — and more specifically to the island of Amorgos from which it takes its name. In its natural range, the plant colonizes dry rocks, arid slopes, and coastal garrigues in full sun, in a hot Mediterranean climate with very little rainfall in summer.
The cultivar forms a dense, spreading bush, 30 to 50 cm in height, with woody stems at the base, covered with persistent, oblong foliage, of an intense and velvety silver-gray, remarkably luminous. It is one of the purest silver foliages one can find among rock garden plants, and it constitutes a constant attraction even outside of flowering.
The inflorescences consist of numerous small globular capitula, tightly packed in rounded corymbs at the top of the stems. Their chromatic evolution is one of the most fascinating features of the cultivar: initially a deep crimson red in bud, they gradually lighten as they open to reveal a bright yellow central disc, passing through salmon pink and pale pink hues depending on their stage. The scarious bracts persist long after flowering. In its natural environment, the species blooms from June to August. In cultivation, 'Ruby Cluster' blooms from June to September.
It requires full sun, a very well-drained soil, dry to moderately fresh, poor to ordinary, and fears stagnant humidity in winter. Its hardiness is limited; in regions with cold and rainy winters, cultivation in a pot brought indoors is preferable. It naturally integrates into Mediterranean rock gardens, walls, and gravel gardens.