Tip for emerald owners: clean your emeralds with a soft chamois or other cloth, using warm soapy water if needed. Do NOT use steam or an ultrasonic cleaner. Most emeralds are oiled and/or fracture filled and steam and ultrasonic cleaners can cause damage.
There are a lot of varieties of quartz on the market but there are also varieties of so-called quartz that are not actually quartz or are altered quartz. Your seller SHOULD be up front and tell you what you’re buying.
Typical quartz varieties include:
quartz (clear, colorless)
milky quartz (translucent white)
rose quartz (pink/milky pink)
amethyst (purple quartz)
citrine (yellow/yellow green/yellow brown quartz – often heat treated to get golden/orange colors; natural is dichroic)
Carats have been the standard for weighing gems since ancient times. “The name is derived from the see kuara of the African Coraltree or from the kernel (Greek keratiton) of the Carob bean.”  One carat is a unit of mass equivalent to 200 micrograms, 0.2 grams, 0.007 ounces, or about the same as a standard paper clip.
Both cut and stone density affect the total carat weight of a stone. For example, I sampled five 5mm round sapphires and the carat weight varied from 0.615cts to 0.835cts as a result of differences in how each stone was cut. To compare to other gemstones, I found a 5mm round andalusite weighing in at 0.45cts, kyanite at 0.6cts, and a brilliant cut diamond at 0.5cts.
One thing to note as you are looking at gemstones is that larger stones are more rare and will likely cost more per carat than the more easily obtainable, small stones.
 Gemstones of the World: Newly Revised & Expanded Fourth Edition by Walter Schumann, p30
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”)
Because amethyst is one of the more popular stones, it seemed worth learning a bit more of how it was used throughout history. As the excerpts below highlight, amethyst was used for more than preventing drunkenness.
From A Lapidary of Sacred Stones by Claude Lecouteux, p47-48
“In the thirteenth century, it was said the amethyst prevented the devil from causing harm and preventd a person from seeing ‘ghosts’ (fantasme). It also provided protection from the entity known as a nightmare and from fevers, it granted riches, and made one humble, courteous, and gracious.”
“Etched with the moon and sun and hung around the neck with hairs from a cynocephalus* and feathers from a swallow, it protects one from evil spells. Its magical properties are increased if set in gold or silver and if a man on horseback holding a scepter is carved on it.”
* cynocephalus likely refers to some type of baboon
“If one finds an image of an amethyst of a man with a sword in hand seated on a dragon, and this stone is then set in a ring of lead or iron, the wearer will obtain the obedience of all the spirits, tnd they will revela where treasures are hidden and answer whatever questions he may ask.”
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