Talk to the guy. He will give you a plinth and a replica statue piece for your POH. You can then install it in the Study in you house but you can't add the replica piece because you need its twin, just like the statue you have just completed at the museum. Our Runescape Gold service will be your best choice when you need gold.
Er, didn't it explode in front of you yet? Anyway, you have the next 29 weeks of repeatedly rebuilding that same museum statue, watching it blow up and getting a new replica rock for your own POH statue in Runescape. Each twin set of rocks added to the museum statue yields the same amount of experience as it did the first time.
You can only rebuild the statue completely once a week. You can however get a head start on next week's statue by continuing to train your skills and getting twin rocks from each. But you'll only be able to add 28 of them (14 twins). The museum guy will stop you from adding the last pair until one week has past since the last one was completed. We recommend you to buy the cheapest runescape money service from our site.
So, 900 rocks total, 30 completed museum statues in 30 weeks (including this one), all in order to have a completed statue in the Study of your POH. What will be the grand reward after all that? No one knows. They're keeping us in the dark. Our guess is because if we knew now, we'd probably right it off as not worth the trouble.