I always thought that the Dreamstone's rarity was due to Zeal Palace's over excessive use of the stone. Think about it; Zeal used the Dreamstone aka the Red Rock to create many things such as the Pendant, Amulet (Janus' Amulet is made of Dreamstone last I heard), the Epoch, the Mammon Machine, Masamune, Red Knife, etc. It was used to keep the Zeal Palace Continents floating. And before that it was used as currency. Both Melchior and Gaspar mention how Dreamstone was once used as currency when you are trying to reforge the Masamune. Just talk to Gaspar when you are trying to find the Dreamstone to repair the Masamune and he'll mention it. So will Melchior when you speak to him and show him the piece of the Masamune.
I also noticed something. Lavos absorbs the Planet's energy, Dreamstone absorbs Lavos' energy, Dreamstone is from the Planet...Lavos draws the Planet's energy, Dreamstone draws Lavos' energy, Dreamstone is from the Planet....notice a circle here?
Another thing to take into consideration. The Planet is the Entity right? Well, Dreamstone came from the Planet, which means it came from the Entity right? Well, the Entity is capable of dreaming, and the Red Rock, which is the Dreamstone, gave man (humanity) the ability to Love, Hate, and Dream. Notice how Love and Hate were born and the ability to Dream all came from that Red Rock (Dreamstone) that man (humanity) found that came from the Entity (the Planet) that is also able to dream and possibly Love and Hate? If that made any sense, which I hope it did, do you guys see any connections here?