Among the options in material for garden art, pathways, walls, urns and the like, gray or bluish stone is appealing for its natural look. To help stone items blend even more cohesively with the garden as a whole, incorporate some silver- or blue-toned foliage into your garden. They will echo the color of the stone and act as a link between it and the whole planting.
A few plants to try: lamb's ears (Stachys byzantina), sea kale (Crambe maritima), blue fescue grass (Festuca glauca 'Elijah Blue'), and any Verbascum or Artemisia. Want something edible? Some artichoke varieties have bluish leaves. For shrubbier plants try Fothergilla gardenii 'Blue Mist', which has bluish summer leaves. Normally we relate false blue indigo (Baptisia australis) with blue flowers, but many selections also boast bluish- or grayish-green leaves.
Image, public domain.