The Rough Astral Diamond refining limit serves as baseline for how much ADs a you can earn. Any character on any item level can earn RADs using the Random Queues or other dungeon runs. Only 100,000 of those however can be converted into “real” Astral Diamonds per day. With this hard cap the devs control how much free Astral Diamonds players can add to the economy. Once at level 70, you normally shouldn’t have any issue hitting the cap, but lower levels might not get there consistently.
There are two main way of accumulating RADs on characters. The first one is running the available Random Queues. The Leveling and Intermediate Queues are a must, but depending on your toon’s strength it might not make sense to do the Advanced or Expert ones.
After completing your random queues, you should still have a few thousand RADs to go. That’s where method #2, so called “farm runs” take over. Players use the fastest dungeons (currently Temple of the Spider, Shores of Tuern, and Demogorgon) to quickly farm end chests and the RADs that come with it. Unfortunately this is more viable if you’re VIP. Because the daily additional re-rolls mean you will like get more profit out of chests. Doing weeklies on alts can also contribute a lot towards the daily limit.
Another way of making a few ADs is by participating in events. The bigger seasonal ones offer unbound rewards that can be flipped for currency on the Auction House. Since most entry requirements are low, all levels can often fully participate, and make as much ADs as veteran players. Unfortunately great AD-making events are rare, and the items get devalued rather fast. Call-to-Arms companions for example used to sell pretty well, but merely go for pennies nowadays. The longer rewards exist, the less valuable they get.
A general advise in terms of events is to stash away the items for a while. During the event days supply gets up and prices go down. Waiting a couple weeks or even months could net you significantly more.