Q: I would like to carve some pieces to place in my garden. What types of stone can be placed outside?
A: Depending on your garden and your expectations, nearly every sculptural stone can be placed outside. But note the qualifications at the beginning of that sentence. The reaction of various stone to the environment can include dissolution, loss of high polish and fine detail, bleaching by sunlight, iron staining (rust), algae growth, staining by waterborne chemicals, and disintegration caused by crystallization and expansion of salts and ice within the stone.
Moisture is the single most detrimental factor, whether from rainwater which is naturally slightly acidic, soil moisture wicking upward from the ground, or sprinkling systems repeatedly dowsing the sculpture.
Alabaster (gypsum) and anhydrite are soluble but they can last longer than you may think. I have had an alabaster piece exposed outside in Portland weather for about three years with very little effect. (Dissolution does suggest artistic possibilities, however. I recall some fascinating transformations as ice sculptures slowly melted at Camp Brotherhood a few years ago.) Marble and limestone will dissolve so slowly that the only effect you may see in your lifetime is dulling of a high polish.
Soapstone, although soft, is very stable chemically. It has been used for centuries as a durable building stone throughout Scandinavia, but it may contain pyrite, which can cause iron staining.
Stone porosity, including holes in travertine, spaces between grains in sedimentary rocks (e.g., sandstone, limestone), or fractures in any rock type, can be an important factor. If a stone can soak up moisture it can also soak up gardening and lawn care chemicals resulting in white coatings on the sculpture surface and possibly providing additional nutrients for algae. But then again, what are your expectations? I have a piece carved of porous volcanic tuff, sitting directly on the ground. In the dry summer it’s bright yellow and red, but lush green all winter when it’s wet. It’s a sculpture for all seasons.
Granite and basalt are very durable although some contain reactive iron-bearing minerals. Many sandstones, limestones, and marbles are good choices, especially if they won’t be water saturated. Avoid black marble and limestone since they quickly bleach or develop white coatings.