Here is some additional information about gardening with plants.
GARDENING OR OTHER OUTDOORS STUFF WITH STONES AND CRYSTALS
First here is a brief list of some of the stones that are sun sensitive and may change color or be effected by the heat or light of the sun. This doesn't mean you shouldn't use them, only that you may want to keep an eye on them for what appears to be progressive damage. Many stones are expensive because of their exquisite beauty, their delicacy, and their rarity, so you'll have to use your own judgement about what kind of stones you are willing to risk losing.
I'm sorry this won't be alphabetical as I'm just spinning this off the top of my memory banks right now.
These can (but might not) lose or change color: Alexandrite, ametrine, amethyst, gem spudomene, dioptase, kunzite, hiddenite, lepidolite, smokey quartz, topaz, blue calcite, citrine, chrysoprase, opal, rose quartz, tangerine quartz, golden healer quartz, malachite, azurite, rutilated quartzes, tourmalines, flourite, peridot, turquoise, vivianite, proustite, silver colored metals, mercury based minerals, arsenic based minerals.
Pearls and mother of pearl shells will deteriorate rapidly and crumble from either water or sun. Same goes for stilbite, apophyllite, corals, opals, and any crystal with tiny, fragile needle-like points or rays.
Some of these, like the mercury or arsenic based minerals, you wouldn't want to put in an outdoor garden anyway. The stones that I'll be recommending are the ones that I feel would not be toxic to the garden environment. If you find a crystal that you are unsure of how soft it is, or if it may contain considerable toxins, I would suggest a book such as "The Audubon Field Guide of Minerals" plus there are other valuable geological/mineralogical information resources at libraries, rock shops, book stores, lapidary clubs, the internet. All you need to know is the name of it or show it to a pro who can tell you how to identify it.
You may spot some repetitions here, as some of these are included in the above list and can change color, but here is my list of stones that I feel would be safe and good to use in your garden. And keep in mind that many of these can be found growing from other common rocks that are scattered about the land, roadsides, creekbeds, rivers, beaches, etc.
Agates (all agates), amethyst, ametrine, amazonite, ammonite, aquamarine, aventurine, apache tears, apatite, bloodstone, blue lace agate, calcites (all colors), carnelian, citrine, pure copper, diamond, emerald, fire agate, flint, flourite, garnet, gold, golden healer quartz, granite, graphite, hematite, herkimer diamond, iron, jade or jadeite, all jaspers, lapis lazuli, malachite, moonstone, moss agate, peridot, petrified wood, pyrite, rose quartz, ruby, sapphire, silver, smokey quartz, sodalite, spirit quartzes, tangerine quartz, tiger eye (all colors), tiger iron, topaz, zircon.
Also look for bigger rocks from outdoors that are unique in appearance, color or pattern. Big stones that are rounded rather than sharp or straight edged promote better movement of Chi through the garden, and they are also more comfortable to sit on. Flat pieces of slate are marvelous for stepping stones and they glisten all colors when wet. When stood on edge they make lovely dividers between sections. Many rocks are encrusted with glittering bits of quartz, pyrite, mica and are very reflective of rainbow light.