Jade is the gemstone name for two different mineral forms, Jadeite and Nephrite. These two minerals can be identical in appearance and are similar in their physical properties. Until modern times no distinction was made between the two different types of Jade. As scientific methods improved it was discovered that, however similar they appeared, their mineral compositions were different. Both minerals are silicates, but Jadeite is an aluminum-rich pyroxene of tightly packed microscopic crystals, while nephrite is a magnesium-rich amphibole composed of extremely dense mineral fibers that are interlocked and very tough.
What does that mean in English? Nephrite is slightly softer (6-6.5 Mohs) and more prone to scratches, and Jadeite (6.5-7 Mohs) is not as dense and is more prone to chipping.
Both are beautiful. Both are valuable. There seems to be an attitude lately that Nephrite is not AS valuable as Jadeite. I think that difference is based more on availability. Jadeite is harder to get, because the main source is the trade embargoed Myanmar. Nephrite comes from China, Russia, Brazil, Wyoming, Alaska, and California. 
Now you know the two stones, so let's navigate the 'grades' of jade and what they stand for.
Type A - natural, untreated, undergoes a traditional process (plum juice washing and polishing with beeswax), no “artificial treatments” (e.g. high temperature or high-pressure treatments), no color enhancing. (see picture above)
This is the good stuff and harder to find than you might expect {industry reports less than 10% of the material on the market is A grade}. It will always be extremely expensive and most reputable dealers will supply an infra-red spectrometer certificate- but more on that later...
Type B - Chemically bleached to remove impurities, injected with polymer with the use of a centrifuge to enhance translucency, covered with hard and clear plastic like coating, subject to instability and discoloration over time because polymer gets broken down by heat or household detergent, still 100% real jade with 100% natural color.
This is the majority of jade on the market, according to industry reporting. This includes overseas markets, so watch what you buy on vacation too. It isn't bad, as long as it has been disclosed and you are buying the piece to enjoy, not as an investment.
Type C - chemically bleached, dyed to enhance color, subject to discoloration over time due to reaction with strong light, body heat or household detergent. Cheap souvenir, mostly carved into figurines and low end bangles.
Type D - This stuff is not stable, and usually the bleaching has so weakened the stone that it takes very little to break. Not only is it brittle, but there have been reports of acid burns from the bleaching agent leaking out of the stone over time. I suspect this grade is made up of material that did not respond well to the initial bleaching process {remember type b} and was over-treated in an attempt to get the discoloration out.
