Sometimes this is done during the upkeep, or in response to card draw, in case your opponent draws a split second card (say, Krosan Grip for your Icy Manipulator), countermagic, or an untargetability effect, but in general it's best to wait until the last moment
Thus, an ideal home for Hall of Gemstone is in mono-green Commander decks
Greninja ex Special Illustration Rare Twilight Masquerade Before Surging Sparks came around (and the Pikachu ex), Greninja ex SIR was the top chase card of the Scarlet & Violet era of Pokemon TCG
I encourage you to poke around with the websites search feature and see if you can learn anything interesting about your local Magic scene
Then you put the top six cards of your deck facedown in the Prize Card area without looking at the cards
Thats right, an article about the state of Standard