You don't so much cast a miracle as request one. You state what you would like to have happen and request that your deity (or the power you pray to for spells) intercede.
A miracle can do any of the following things. - Duplicate any cleric spell of 8th level or lower. - Duplicate any other spell of 7th level or lower. - Undo the harmful effects of certain spells, such as feeblemind or insanity. - Have any effect whose power level is in line with the above effects.
Alternatively, a cleric can make a very powerful request. Casting such a miracle costs the cleric 25,000 gp in powdered diamond because of the powerful divine energies involved. Examples of especially powerful miracles of this sort could include the following:
- Swinging the tide of a battle in your favor by raising fallen allies to continue fighting. - Moving you and your allies, with all your and their gear, from one plane to a specific locale through planar barriers with no chance of error. - Protecting a city from an earthquake, volcanic eruption, flood, or other major natural disaster.
In any event, a request that is out of line with the deity's (or alignment's) nature is refused.
A duplicated spell allows saving throws and spell resistance as normal, but the save DCs are as for a 9th-level spell. When a miracle spell duplicates a spell with a material component that costs more than 100 gp, you must provide that component.