Remixed from a drawing in 'Jezebel's Daughter', Wilkie Collins 1880
Source Firkin
Dark squares with some virus-looking dots in the grid.
Source Hugo Loning
A seamless pattern based on a square tile that can be retrieved in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
Simple wide squares with a small indent. Fits all.
Source Petr Šulc.
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
Pattern #100! A black classic knit-looking pattern.
Source Factorio.us Collective
A seamless background pattern with impressed gray dots.
Source V. Hartikainen