What we need is:

-translucent white clay (I usually use Cernit’s translucent because it’s easy to smooth before it’s cured) I prefer “dirty”  clay . . .that is, white clay which I manipulate when my hands are dirty from handling other colours of clay (I think we've all made this kind of mistake). This lightly coloured clay makes the opals look more “natural”.

-opaque white clay

..."Handle" a few different colors of clay to get some colours on your hands.  Then manipulate the translucent clay in your hands a bit to colour it.  Stop when you have the amount of colour you want.

...Feed the translucent through the pasta machine on a really thin setting. . .  Cut the sheet in half.

...Put some flakes on one sheet (not too many flakes since you still want to see a large part of the clay).

...Then put the second sheet of clay over the first, and press down well.

...Take this “sandwich” and feed it through the pasta machine on the thinnest setting.

...Lastly, put this sandwiched sheet on a sheet of opaque white clay and press down well. 

...Cut or form the clay into the shapes you need, and smooth the borders.

...Let them rest until they are quite firm (possibly overnight).

..."Polish” your opals with a small quantity of talcum powder (or cornstarch) by rubbing in circles with your finger until they feel smooth (don’t worry if they seem to be full of powder, you can rinse them after they are cured). 

...Now your opals can be cured, or you can embellish them first for jewelry pieces (I've used ropes of clay to surround them).

if you want to see others opals, see here