Pick for the stage the friendship is in. Rose quartz and carnelian are the stones crystal practice points at for making friends. Lapis lazuli, blue lace agate and amazonite are for the conversations that keep one going.
A friendship stone is the only stone in crystal practice you might sensibly buy two of. Everything else on the shelf is bought singly, for one person, and stays with them. This one has a second copy and a handover built into it, and that is what makes it a different kind of purchase rather than a rose quartz with a different word attached.
It also has a limit that goes unsaid. Your friend did not choose the stone, was not asked, and may never learn any of this happened. Whatever a friendship stone does, it does on your side of the friendship, on whether you go first and whether you say the true thing rather than the easy one. Those happen to be the only parts you had any say over anyway.
The paired stone, and what to do with it
Buy two of the same stone and keep one. Tumbled pieces are the practical choice here, since they are cheap enough to buy in pairs and survive a pocket. Rose quartz is the usual pick because crystal practice treats it as the stone of love in every form rather than the romantic form only, and carnelian is the one for warmth and nerve and being the person who suggests something.
The handover is the part people overthink. Say why you picked it, in your own words, and stop there. Something along the lines of one each, I thought of you, does more than a paragraph about properties, and it lets your friend take the object at whatever level suits them. If they are not a crystal person, leave the framing out altogether. A small stone given on a particular day works perfectly well as an object with a date attached to it, which is most of what any keepsake has ever been. The wider etiquette of giving one, including the old question of whether you should choose a crystal for somebody else at all, sits in what crystals make good gifts.
What you do with your own half afterwards is the quiet part of the practice. Keep it where you will meet it, in a coat pocket or a bag, and let it be the prompt to send the message you have been meaning to send. A pair only means anything while both halves are in use.
If there is nobody to hand the second stone to yet, the single-stone version of this stage is a placement rather than a carry. Crystal practice puts rose quartz near a front door or in the room where people sit, on the reasoning that you are opening a space rather than working on a person. Carnelian goes in a pocket instead, since its half of the job is the going out and the suggesting, and it does that from closer in. Neither is a substitute for turning up somewhere twice, and that part of making friends has no shortcut in it.
Carrying one into a conversation you have been putting off
The other stage is a friendship that already exists and has something stuck in it. Choose the sentence first. The single thing you have been avoiding saying, in the words you would use out loud, decided before you leave the house. Then pick the stone up, hold it, and say that sentence to yourself once. Then put it in your pocket, or on the table between you when you get there.
The stone is doing the job a wedding ring does when someone turns it, or a coin in a pocket. It gives your hands somewhere to be and it reminds you what you came to say. Lapis lazuli is the one crystal practice associates with truth and loyalty, which puts it at the front of this queue. Blue lace agate is the gentler option, pointed at the anxiety that stops the conversation happening at all, and amazonite is associated with saying what you mean without either shutting down or overshooting, which what chakra amazonite belongs to goes into.
One thing about lapis in particular, because it decides how you carry it. Britannica describes lapis lazuli as a rock rather than a mineral, coloured by lazurite and usually carrying white calcite and small gold specks of pyrite that look like glitter. At 5 to 5.5 on the Mohs hardness scale it is soft for something handled every day, and minerals.net notes that it chips or cracks easily when knocked and that it is slightly porous. On a keyring it will not survive long. In a coat pocket on its own, or set on a table, it is fine. If you want to know what any of the five is made of, each has a page in the Crystalance Mineral Library.
The conversation still has to happen, and no stone gets you out of that. What a stone in your pocket does is give the fifteen minutes beforehand something to do with your hands, which is a smaller thing than the crystal shelf promises and a larger one than it sounds.
