It's a fact that many ornaments benefit from the ageing process, developing a patina that adds to their character, and cleaning these would be inappropriate. But others look better when they are pristine and the cleaning regime is the same for quarried, cast or reconstituted stone.
Wiping over with a soft damp cloth at regular intervals, perhaps even daily, is recommended, and this will prevent grime from collecting. But how many of us have the time or inclination to keep to a task that will eventually become rather arduous?
Dedicated stone cleaners and algae removers are available, but monthly or bi-monthly cleansing using a cloth, sponge or soft brush with warm water that contains a mild detergent such as washing-up liquid, followed by a cold water rinse, will normally suffice.
What is essential is that any solution that's used should be non-abrasive and non-acidic i.e. pH neutral, and this of course precludes bathroom cleaners and vinegar, the latter is widely used as an effective cleanser but owes its efficacy to the presence of acetic acid.
The reason why acidic materials and abrasive detergents need to be avoided is that they scour the ornament's surface, increasing both the stone's porous nature and susceptibility to the kind of staining that is difficult, if not impossible, to remove.
During periods or harsh weather, particularly over the winter, you should take note of the the manufacturer's recommendations regarding susceptibility to frost damage. Extra precautions may have to be taken, and this is always the case if you have a water feature.
You should also bear in mind that, although many ornaments will be unaffected by sub-zero temperatures, their appearance may suffer from collecting more debris, such as falling leaves, than during the summer.
So fitting a weatherproof cover, made from a strong and durable material such as polyester with a PVC lining, would seem a good idea. Covers are available for many ornaments with drawstrings to ensure complete protection even during heavy winds and storms.
Self-contained water features need extra attention as serious, possibly terminal, damage can be caused by frozen water to the circulating pump and other internal parts even though the exterior stonework might remain unaffected.
So use the dedicated outlet to drain all the water out of the feature before winter arrives and if possible take this opportunity to remove the pump so that is can be cleaned of any detritus that has built up over the summer.
Any algae and limescale will have a detrimental effect on the pump's performance, so these should be removed with a mild washing agent, paying most attention to any moving parts such as the impeller.
In the spring, with all the mechanical elements replaced and when there is no danger of freezing weather, the ornament can again be filled with fresh water.