Gemstones and mineral specimens have many properties. One that is particularly important for jewelry is hardness. Why? A stone that is soft should not be used in an unprotected setting or in a piece that is likely to be bumped.
For example, I had a lovely opal ring with four opal cabochons in prong settings. Not realizing at first how fragile opal can be, I wore the ring regularly. It didn’t take long before I’d chipped one of the stones. I was slightly more careful with it but still managed to chip a second stone and lose that first one (as it was loose in its setting). Fortunately, we were able to match up the opal, cut and polish new cabochons and restore the ring completely.
For stones that are below 7 on the Mohs’ hardness scale, dust can do some damage if rubbed into the stone as quartz particles are fairly common in dust. To help identify how hard a stone is, there is a “simple hardness tester” that can be used as a guide and to provide an idea of what the various scale values mean along with an example in parenthesis:
1 (talc) and 2 (gypsum): can be scratched with a fingernail
3 (calcite): can be scratched with copper coin
4 (fluorite): easily scratched with knife
5 (apatite): can be scratched with knife
6 (orthoclase): can be scratched with steel file
7 (quartz): scratches window glass
8 (topaz), 9 (corundum), 10 (diamond) : no simple test
I would not recommend actually trying any of the scratch tests on finished stones but rather use the information to help understand what the various Mohs values represent. Note that the scale is relative, so that the difference between a 4 and 5 on the scale is not the same as the difference between 9 and 10.
 Gemstones of the World: Newly Revised & Expanded Fourth Edition by Walter Schumann, p20 (based on the table “Relative and Absolute Hardness Scale”)
Fordite (also called Detroit agate or motor agate) is made up of layers of hardened automobile paint from the oversprayed paint when people used to hand spray paint onto cars as part of the manufacturing process. When the paint got hard enough, this enamel material could be cut and polished. These “stones” have a variety of colorful designs and are often cut into cabochons to be used in jewelry.
Rainbow calsilica’s history and origin is less clear. Like fordite, rainbow calsilica is known for having a variety of colors and patterns. One supplier claims the material comes from mines in Mexico. The Gemological Institute of America lab tested the material and states it is man-made. Another lab, SSEF in Basel, Switzerland came to the same concluion. While lovely, rainbow calsilica “[c]onsists of dyed sand that has been firmed with artificial resin.” 
 Gemstones of the World: Newly Revised & Expanded Fourth Edition by Walter Schumann, p282
Feldspar comes in a lovely variety of stones: moonstone, labradorite, orthoclase, amazonite, sunstone, and spectrolite.
Moonstone displays adularescence, a blue-white opalescence which is a result of light interference with the layered structure of the stone. The most striking example of this is in rainbow moonstone.
Similar yet different, labradorite and spectrolite display labradorescence, “iridescence in metallic hues, called schiller”  with blue and green colors commonly seen, although this effect occurs in all colors.
Pink moonstone and sunstone can also display asterism, typically with four rayed stars.
With all the varieties and colors, feldspar is one of my favorite minerals!
 Gemstones of the World: Newly Revised & Expanded Fourth Edition by Walter Schumann, p.53
Asterism is a term which describes “the effect of light rays forming a star (Latin aster, star)…It is usually created through reflection of light by thin fibrous or needle-like inclusions that lie in various directions.” 
Stars in gemstones may have four, six, or (less frequently) twelve rays. With these stones, the angle of the stone to a light source is important to be able to see the star. Natural gemstones known to exhibit asterism include:
star rose quartz
star blue quartz
 Gemstones of the World: Newly Revised & Expanded Fourth Edition by Walter Schumann, p.52