Received 13 August 2008; revised manuscript received 27 January 2009; published 23 February 2009 We generalize the theoretical model of magnetic metamaterials made of dielectric particles to treat random particle sizes, shapes, and orientations. We demonstrate that the magnetic-dipole response of these randomly shaped subwavelength particles can be approximated with Mie theory as though they were spheres, while a quasistatic ellipsoidal approximation is used for the electric-dipole response. We verify our model with experimental measurement of a bulk magnetic response in a micropowder of milled SiC particles. Using such a crude powder could lead to immensely simplified negative permeability inclusions for negative index metamaterials. There has been rapid progress recently in the fabrication of infrared and optical metamaterials. These are artificially structured composites with constituent elements much smaller than the wavelength of light. Metamaterials may possess electromagnetic properties not found in nature, such as a negative magnetic permeability or, by the simultaneous pairing of negative permeability and permittivity, a negative index of refraction. These metamaterials allow for applications such as superlensing 1 or invisibility cloaks. 2 Magnetic metamaterials are not only a crucial component in these applications, but are interesting in their own right. The most common approach to fabricating optical magnetic metamaterials is to make the constituent particles out of metallodielectric structures such as split-ring resonators SRRs. More recently, variants such as staples, single rings, U shapes, and paired rods or strips have become preferred for optical metamaterials. 3 Fabrication of these structures is often limited to planar geometries, although recently four planes of SRRs were fabricated by a layer-by-layer technique. 4 Despite these impressive accomplishments, metamaterials composed of metallodielectric structures remain difficult to fabricate due to the intricate metallic patterns and tolerances and the need for sophisticated techniques and equipment. Furthermore, the metallic losses are large at optical frequencies. To avoid these problems, we and others have investigated the theoretical foundation for designing metamaterials using much simpler spherical inclusions. A negative permeability can be formed by a lattice of magnetodielectric or polaritonic spheres. 5‐7 On the other hand, a negative index can be obtained by overlapping a negative permittivity using a second set of spheres 5,7 or a single set of coated spheres. 8 More recently, experimental verifications were reported, either of the magnetic-dipole scattering of single SiC whiskers 9 or of the negative permeability of a cubic lattice of ferroelectric cubes. 10 In this Brief Report, we present a generalization of effective-medium theory for metamaterials made of largepermittivity dielectric particles to include particles with random sizes, shapes, and orientations. We also verify this experimentally with a sample of randomly shaped and oriented SiC microparticles. The sample was made by extremely simple mechanical techniques and is essentially nothing more than very fine sandpaper grit. Despite the randomness, the theory and experiment indicate a bulk magnetic response which is almost independent of particle shape and is hardly degraded from that predicted for an equivalent ordered lattice of spherical inclusions. Also, a bulk electric resonance is found which is considerably broadened by the wide variation of particle shapes. The results, particularly the magnetic response, suggest that such dielectric particles might be a much simpler alternative to SRRs for metamaterial fabrication. We now summarize the effective-medium theory for dielectric-based magnetic metamaterials. A material composed of particles of various sizes, shapes, and orientations may be assigned an effective permittivity eff and permeability eff if the particles are much smaller than the wavelength in the host medium and if the density of particles is not “too great.” In this case an incident field induces in general both electric- and magnetic-dipole responses in each particle. If a composite is made of N particles in a unit volume with permittivity s=ns embedded in a host with permittivity h=nh , then the effective parameters are 8 eff 0