Can anyone tell me an approx value of an 8"×12"×4" quartz crystal cluster, & 1/4th that size green quarts cluster?

Give them a call. They are very helpful

IDK. But they are friendly and will answer your question.

Due to the size it could be profitable. The amount seriously depends on where you are trying to sell it. Honestly, they are priceless. That is pretty big, which means even more amplifier. The shop, Crystal Blue will give you a more precise answer on their scale if you call or go in. They are extremely helpful!