Refractory bricks are used for refractory structures that are exposed to high temperatures, such as cookers and stoves. The stones help keep the fireplace in good condition and also allow the temperature to be distributed more evenly.
Refractory bricks are shaped and fired in a kiln and therefore the stones may differ in size by a few millimetres. The fireplaces are originally designed to cope with this difference. The fireplace parts should not fit 100% as the parts move when they get hot. You may have to adjust the stone to make it fit your fireplace.