Might not be super subtle, but quite original in its form.
Source Alex Smith
This pack of filters can help you adding a blocky overlay to objects. May come handy at drawing blocks of stone.
Source Lazur URH
Inspired by a drawing in 'Kulturgeschichte', Freidrich Hellwald, 1896.
Source Firkin
From a drawing in 'Handbook of the excursions proposed to be made by the Lincoln Diocesan Architectural Society, on the 27th and 28th of May, 1857', Edward Trollope, 1857.
Source Firkin
This is so subtle: We’re talking 1% opacity. Get your squint on!
Source Atle Mo
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
From a drawing in 'Cowdray: the history of a great English House', Julia Roundell, 1884.
Source Firkin
Small gradient crosses inside 45-degree boxes, or bigger crosses if you will.
Source Wassim
If you’re sick of the fancy 3D, grunge and noisy patterns, take a look at this flat 2D brick wall.
Source Listvetra
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
From a drawing of the coat of arms of the Ottoman Empire on Wikimedia.
Source Firkin
A seamless pattern with green and yellow diagonal lines on top of a white dotted background.
Source V. Hartikainen
Heavily remixed from a drawing that was uploaded to Pixabay by ractapopulous
Source Firkin
Stefan is hard at work, this time with a funky pattern of squares.
Source Stefan Aleksić
People seem to enjoy dark patterns, so here is one with some circles.
Source Atle Mo
A seamless design of flowers remixed from a jpg on Pixabay by Prawny.
Source Firkin
Prismatic Curved Diamond Pattern No Background
Source GDJ
Dark, lines, noise, tactile. You get the drift.
Source Anatoli Nicolae
Horizontal and vertical lines on a light gray background.
Source Adam Anlauf
ZeroCC tileable stone texture, edited from pixabay. CC0
Source Sojan Janso
Tile-able Dark Brown Wood Background. Feel free to use it as a background image in your designs or somewhere on the web. By the way, the color seems to be close to Coffee Brown.
Source V. Hartikainen