From what I've learn, we can do scrying from just anything. From a Crystal, the stone that you mention, the sky, the river, from anything that you can think of, even from a tree. But for the way to do it, I think the key is just relaxation~ *Correct me if I'm wrong~*
Yes can do you scrying from anything that you feel comfortable with. I mainly do water and cloud scrying the most, But other times i tend to scrying with many other objects as well.
All you need to go is go into deep relaxation and most people can go into a trance and start seeing images that come into the mind. All always go into a trance automatically sometimes and so i start see images come through and start reading them. But there are methods to do scrying. You can also do scrying with candle flame. I have never used this technique though.
I think you should really look more into meditation, relaxation and scrying. These can really help you.
As stated before it is all about preference (pretty much all of Magick is). You ask specifically about Obsidian, which makes me think that you have it and are inquiring about it. If that's the case (and in any case), it's good to get to know the energy you are working with when scrying. Get to know the obsidian. Ask questions and listen, or simply feel the responses. Some crystals are said to have preferences as well, meaning that some are willing to work with you and others are in line with different energy. Never force your will on anything and always be respectful.
As for actual scrying, you can use a mixture of what you see physically and what subconscious (psychic) impressions you get from the crystal (or energy in use). I scry people and get impressions from the vein patterns on the backs of their hands. When this happens, I view their energy in my own light, so the meaning often comes to terms when both of us begin to understand it (the meaning has ground within the both of us and sometimes only to us).
You can also use your other senses, both physical and subtle. Try to work with all.