If you have a hammer and hardie and want improve your skills then a good way is to get standard 300mm x 300mm x 10mm (thickness) marble tiles from a tiling store. The best ones to go for are Botticino (off white) but just see what they have as the more different types of stone you use, the better.
Using the hardie only break the tile over it into 2. Continue to break using your hands until you get down to pieces about 20cm x 20cm.
Now you use the hammer as well. It’s very important at this stage that you continue to cut across the centre of the fragment. Don’t try to cut your tesserae until you have one side of the fragment a maximum of 4 -5cm. Try to cut too soon and because the piece is too big you run the risk of the cut not going in a straight line.
When you have one edge small enough cut strips 8 -12mm in width. Then cut these strips into 8 -12mm cubes. Always look for a cube of your working size within any fragment you cut. Cut this first and you will find that you naturally have triangles and 1/2 cuts (10mm x 5mm) as a byproduct. It’s very important that you develop the knack of ‘seeing’ your tesserae in an uncut piece. It takes time but it may help you if you have a couple of already correctly cut tesserae of the right size in front of you to refer to.
With a marble tile 300mm x 300mm x 10mm which weights about 2.5kg you need to aim at having 200 -300gms in waste. The rest should be usable cut tesserae. Don’t worry if you have a lot of scraps at first, it does take time to get your skill level up. Always remember, ‘Cut across the centre’. Keep this is mind until you get to pieces with at least one edge of 4 – 5cm. Try to cut tesserae from too large a fragment and you just end up with lots of waste.
A word of warning: you will cut pieces that you’re not sure you’ll be able to use or not. Decide then if they can be used, if you’re unsure then chances are they won’t be any good. What you don’t want to do is to to put them in your basket and then end up having to decide if you can use them when you should be focussing on setting your mosaic.