You are correct for Riddles 1, 2, and 4. Sadly, the answer to Riddle 3 is not ebony. Still, a good attempt. And thank you for the numbering; I suspected what you were associating each one with, but I wanted to make certain.
What I gather in the first riddle leads me to believe it is amber. It comes in various colors, mainly represented as a golden substance, was known in ancient times, was believed through the ages to hold mysterious powers, was considered valuable, is a solidified form of a sap of a type of conifer, and has been discovered by many to have within its recovered pieces the remains of different bugs that became subsequently imprisoned--- extinct, ancient bugs.
A perfect amount of reasoning, covering everything I was thinking of when formulating the riddle.
The second riddle leads me to believe coral is involved. Not a plant, but an animal, each part is very small and beautiful; but, gathered together, coral can form some of the most treacherous and sharp reefs known.
Again, excellent reasoning. A few additional bits of trivia: The actual coral polyp (yes, that's what they're called) looks like a miniature sea anemone, and thus, quite flowerlike. And not just treacherous and sharp, but huge also...Australia's Great Barrier Reef was what I had in mind when I wrote that line. Hard to believe something so small can create something so big, huh?
The fourth appears to be pearl. This, too, was highly prized through the ages; and, the East was where many pearls were harvested. Everyone has, of course, heard of "Mother of Pearl" also.
Not just 'were harvested'. According to encyclopedia.com, Japan still (at the time the entry was written, at least) has the lead in cultivating saltwater pearls, while China bears the same for freshwater pearls. And I'm not sure 'everyone' has heard of Mother of Pearl, but yeah, most people have. Another excellent bit of reasoning.
Again, though, the answer to Riddle 3 is not ebony. There's another substance I have in mind, although it's not as well known as the others. Think about what common factors the other three have, and then go looking for something else that fits that description.