Need advice for breaking up obsidian

As said above, MUST have good googles. Thick gloves.

I would recommend a hammer by hand, and try and find something solid enough to sit in front of your legs. I broke up all my obsidian, but even with leather gloves I had pieces stuck into my hand. Small slivers actually. My shins were bloody from my jeans getting small pieces through them - hence my rec for the solid material in front of you. Wear a long sleeve shirt.

If you have a dog food bag or plastic feed bag, throw in it and smack it on a concrete slab, driveway etc. That'll break it, might take a few swings but the bag will help keep the shards from flying.

no

A big chunk is easier to work with when using a hammer.
Dividing smaller pieces in to halves and thirds often damages a large portion of the stock.
Yes, a good application for a saw.

I meant to come post an update. I scored around it as deep as I could with my Dremel where I wanted it to break, put a chisel in the gap, and lightly tapped it with a hammer and it broke right where I wanted it to every time. So that method worked really well.

A few years ago, I purchased quite a bit of obsidian in pieces lemon to cantaloupe size.  I tried various methods of cleaving with hammer and chisel but almost every time it would not go the way I hoped and often would break sideways in a way I didn't want at all.  So I gave up and kept the large pieces to figure out later.

Since then I got a saw and it's so soft and easy to trim any way I want!  It's almost like sawing a piece of hard candy rather than a rock.

If you can find someone or a rock club with a saw it will be time well spent.

Not until I was about to press send on this message, did I suddenly have the thought that obsidian is so soft that one of those carbide-edged flat hacksaw blades might do well with it.

Boy, can I relate to that!  It has happened to me with several other materials.  I have found that sometimes adding some or all of the words "lapidary" "rough" "rough rock" to a search can help.

Perhaps the greatest learning surprise for me after getting into rock tumbling is that glass/obsidian is so soft! All my life, I had just presumed glass was super hard stuff. Every time I had waded barefoot in a river I was always scared of stepping on some broken beer bottle that might have been in that river for hours, or days, or years or decades. But it never happened and I was always puzzled by this even though relieved.

I live in Oklahoma and among several major east-west sand bottom rivers, the Arkansas, the N and S Canadians, and the Cimarron. Since tumbling, I've paid more attention to areas that have more broken glass in them after 4th of July parties and such. Now I've noticed that just days later downstream the glass no longer has hard edges.

And of course in the tumbler I was shocked at how quickly glass and obsidian can "disappear". Then when I got my lapidary saw and cut my first piece of obsidian, it sawed so easily it felt like cutting through a piece of hard candy. Obsidian seems even softer to me when sawing than most manmade glass.