Typically, these gems become more available as a lot of players quit, same with other items like gear. However, as more newer people arrive, old ones come back etc. and they wish to rebuild their characters, these things go up in rarity - and therefore price (Just look at the number of people buying 110% BoEs lately).
Right now the latter is the case, and there's not a lot of OSS hunters, while the main guilds tend to sell gems inside the guild. These 3 main sources (CA + DW; OSS and people's gear) have been in decline lately, therefore the prices go up. As for BD gems, they are mostly sold inside the guild. The only time they'll appear on the market is when a pro player (or more) quit(s).