Composite plastic compositions based on the dispersion of selected crosslinked polymers in certain thermoplastic matrices are disclosed. The composite compositions are readily formed and processed by thermal extrusion processes versus conventional casting processes. Preferred composite plastic compositions based on crosslinked poly(alkyl (meth)acrylate) polymers and modified poly(alkyl (meth)acrylate) thermoplastics are especially useful in the preparation of synthetic architectural materials having a mineral-like appearance, such as that of granite.