Da Crusha:
Im trying to paint obsidian on my bases so I was wondering if anyone has had success doing this. Im thinking about painting it Chaos Black, then Black Ink and maybe a white highlight on the edges. Tell me what you guys think.
Baggronor:
Obsidian can have a whole load of unusual colours within it, its never actual black (same as anything else in nature).
I would try:
Base coat of black with a small amount of green/brown mixed in just to kill the blackness (pure black will stand out far too much and look unnatural). Highlight sharply the tips of the extreme edges with white with a tiny touch of black and green mixed in (again, just to kill the whiteness a bit). Gloss varnish.
Pyro Stick:
I would paint obsidian by painting the rock or whatever chaos black and just varnishing it with gloss varnish.
Ronshank:
As weird as it sounds i’d water down a dark silver based metalic colour and possbly add a few drops of black to it and use that for high lighting. I have quite a few bits of Obsidian at home and it has a dark metalic sheen to it I’m unsure how to highlight it to get that effect exactly so i’d be too scared to do it myself but I am with Baggronor the main thing is that is is not 100% black and has a brown-y to grey-ish hue.
Pyro’s idea using Baggronor’s brown-green mix into the black would probably work quite well and be alot less hassle if you are going to paint a large amount of bases. Having said that Chaos Black with the vanish would contrast with your army a whole lot better as it is already so brown. I’d give them a test run and see what you like better. I have a feeling a shiny pure black might suit your models more, but then it’s up to your discretion which looks best
I tried the black ink on chaos black thing before but it didn’t workout that well for me but I know your painting skills with ink exceed my own so it might just be me but i’d give the others a try first. I hope the bases turn out well. Are they for your Bullcentaurs?
Loki:
I would do it the same as Pyro 
phierlihy:
I’d mix Black in with some dark brown to kill the pure blackness effect. Wash with deep green. Wash again with Black. Highlight the edges with a medium-medium dark brown.
Old One Eye:
i do a base of chaos black then on any of the light catching edges i do a one part liche purple to two parts skull white (1:2)
usually works for my obsidian
Golder Goldeater:
How about : chaos black basecoat and varnish?
It will be black and shiny just like the obsidian (easy too) 
Da Crusha:
thanks for all of the suggestions guys. I started with painting the bases of my cd warriors with black ink, mostly because it was way quicker than using chaos black and gave a similar result. I also tried my first attempt at lava using ThommyH’s WoH tutorial, Im pretty happy with the result. Ill post a pic soon.
Im starting to think obsidian might not be the best idea to paint on the bases of the entire army. I was thinking about ash or some kind of chaos dwarf wastelands, or typical volcanic region terrain. not sure exactly. I suppose black mostly with grey highlights.
IzNoGood:
Hi
Tamiya Colour (from pretty much any decent model store) do a variety of clear acrylic glazes including blue, green red, yellow and black. A couple of coats over white or metal gives great effects. I do warpstone by painting the object black with white lines and coating with the green glaze.
thanks for all of the suggestions guys. I started with painting the bases of my cd warriors with black ink, mostly because it was way quicker than using chaos black and gave a similar result. I also tried my first attempt at lava using ThommyH’s WoH tutorial, Im pretty happy with the result. Ill post a pic soon.
Im starting to think obsidian might not be the best idea to paint on the bases of the entire army. I was thinking about ash or some kind of chaos dwarf wastelands, or typical volcanic region terrain. not sure exactly. I suppose black mostly with grey highlights.
Da Crusha
Servius:
Bagg’s Version works well… I took similar advice with my original CD army… And it looks pretty good. It does however need a pretty liberal amount of gloss… i did 2 brush coats.
Thorne:
Personaly I would use black drawing inks as a base on the part you are to paint and then liberaly use streaks of blood red codex grey dark angel green and enchanted blue allowing it to run into the inks whilst still wet. As Obsidian is natural glass so it will retain not only a shape edge but a glossyness that other minerals do not seam to have. Personaly if I were to do such a project I would make the object a high gloss finnish too