1.I believe you meant what we called as 'the black triangular'. It is formed when your bone and gum shrink, and if the process continue down the time, eventually your teeth will be shaking. There are few minor oral surgery that can make the bone and gum regrow, but not everyone is candidate for the surgery, it has to depend on your level of bone. For non-invasive method, i recommend regular scaling to AT LEAST maintain the already shrink-ed gum, rather then let it progressing worsen.

2. Actually our teeth have some amount of transparent, it to make light to pass through and make our teeth nice. It is usually at the edge of our teeth, it is normal. What abnormal is when you notice your teeth has become more and more transparent. It usually due to acidic food, such as orange juice or maybe you have the habit of night grinding ( Bruxism ), which make the teeth thinner.

I think the statement i have put it in a wrong way. It should be if didnt rinse the toothpaste after brushing, there will be increase risk of us swallowing the toothpaste, and it will cause fluorosis of bone. For a teeth to become fluorosis, high amount of floride must be consume at the TEETH FORMATION STAGE, which is before the teeth erupt. The amount of fluoride in typical toothpaste will be 1000ppm to 1500ppm. Even our tap water has fluoride in it , it is below 0.1ppm. Many cases of tooth fluorosis is due to ingestion of these fluoridated water, it has been a debate whether we should put fluoride in our water system or not, but i think the pros wight more than cons
I have completed my fydo, are you a dental officer too??