First, I must LOL because it did NOT come out like I originally wanted. So…flexibility is clearly the primary requirement. 
I was actually hoping for something more glassy and obsidian-like or maybe granite, but not full black, selective gloss clearcoat and buffing to shine and flatten the front but make the distressed edges look rougher. But…not so much.
My painting techniques are all borrowed from miniatures - I had the old real LEAD ones, so yeah, brain cells deathing are. But aside from that I’m really not that great, its just a matter of patience and layers. You just can’t ever get a contrasting darker color into the cracks and crannies unless either a) START with that color as a base, then paint over that being a little less liberal with the paint and not attempting to hit every low spot, or b) you start with your main color then do several color ‘washes’ with very dilute watery paints (on miniatures you even use ink pigments) so the water carries it into the deep spots for you and then you wait for it to dry.
Sometimes it takes several ink washes in different shades and some repair of the base coat before you move on to highlights.
So for highlights…you do a ‘drybrush’ technique on top which is nothing more than getting a tiny bit of paint on a brush you don’t give a crap about and then mashing the crud out of it on a waste piece until its almost uselessly dry…then you just sort of ‘whisk’ it back and forth like you’re just kind of slapping the workpiece, so what pigment is left tends to grab the high spots only. I’ll often use two contrasting colors and put blobs on a waste piece and kind of alternate how much I get of each, or some on either edge of a brush, before I batter it back to having only the little bit left. Drybrushing is extremely wasteful of paint and can’t be rushed at all…if the brush is too wet, you’re just painting not highlighting.
Between bottom coats, base coats, inking, and drybrushing you build up a lot of texture. Like if I wanted to make something look like denim jeans for example: a medium blue for the main color, then an almost purplish wash down into nooks and crannies to look like both shadows and texture, then a very light blue or even white as the highlights.
Lastly variations in a flat vs. satin or even glossy finish can make a difference in the final feel, flat things just look like they’d feel rougher like broken stone, while gloss obviously looks more like it’s been polished, or is metallic, or wet. For cool colors mixing the main color with some black is the quickest way to come up with a compatible ‘shadow’ color, for warm colors use a dark brown instead (e.g. for shadows in something yellow).
In this case I actually started with a dark pewter metallic for my base, A mix of neon green and blue (going for kind of a turquiose) to get the engraving…not sure I’m entirely pleased with that to be frank. Then I washed the edges and all the broken parts with black, and started layering a mix of gray and a kind of mustardy color for the face, highlighting with the same gray with a little white and the mustard again. I think I actually had the intended engraving color too thick and was too impatient because some of that greenish tint ended up spreading thru the gray face, but after a few more layers of highlights and such I decided I could live with it. Then a satin water-based acrylic clearcoat on the flat face only.
One of these days I’d like to get V-carve or something a bit more than just Easel and really take a stab at one of these with nice bevels and all, at which point I’ll give the black granite or obsidian look another go.
Thanks for asking - hope the old wall of text didn’t wear you down. I’ve been chewing more lead while typing so I may have gone in circles a few times.
