Seems this isn't common knowledge.
The Stone of Change is in the Marketplace, beside the Lordsmarch Bank. In fact, it is directly in front of it, just off to the right a bit of the doors.
At this "Stone of Change", you:
Bind and Attune and item. This makes it Bound to Character, and preps it for other Stone of Change related crafting, such as:
Force or Force Critical (1 or 1d4) Damage on a weapon, +1 Dodge Bonus to an Armor/Shield, +1 Alchemical Resistance to certain accessories.
It also contains tiers I-V of the Adamantine Ritual, which allows you to increase an object's Hardness and Durability.
Why am I explaining this?
Well, after seeing an LFM up for about half an hour asking to get onto a Guild Airship with an "Eldritch Stone", I decide to be the nice guy. I'm no "vet", but after playing for a year and a half, I figured I could hook someone up with some knowledge. I send the guy a tell.
"Hey man, I'm not sure which altar you're looking for. What crafting are you trying to do exactly and on what item?"
"Does your guild have altar of ship? Invite me pls."
"Well... I'm trying to figure out exactly what altar you need, so I'm not sure. What are you trying to do?"
"Like... are you trying to add force damage to a weapon?"
"I'm trying to make my weapon tougher pls let me on your ship!"
"Are you talking about the Adamantine Ritual?"
"If so, you don't need on a ship, you need the Stone of Change. It is in the Marketplace, directly to the right of the doors entering the Lordsmarch Bank."
"No that's not it that's where you bind the item I need the Eldritch altar please invite me to ship!!!"
.... And pretty much the conversation continued like that for another ten lines where I was trying to convince him that the "Altar" he is looking for is one and the same with the Stone of Change, which he's obviously already used once. It included him saying at one point something like "I no more than u." When that didn't work, I changed tactics.
"Okay bro, I'm going to do you a solid. Since you don't believe me, I'm going to switch to a toon that has Addy Ritual V on one of his items. You can inspect the item, and I'll tell you the same thing I already have : The Stone of Change."
So, he gets my character name (I don't know why, but he was insistent to have it before I switched), I switch, I send him another /tell hotlinking the item.
"There you go dude. Bottom line, Addy Ritual V. I performed the Binding, and the Addy Ritual, *both* in the Stone of Change in Marketplace. Cool?"
"Please I need to craft let me on your ship for Eldritch Altar!"
He then invites me to his Party. I'm reluctant, but I accept, because I hope it can speed things up.
I get on the microphone, and without taking a break, explain it all over to him again over Voice, wish him a good day, and drop group. He immediately tries inviting me to party again - I have nothing more to say to him, I log off, I come here and make this fancy post.
Moral of the story? I don't know if there is one. Listen to someone who has already finished crafting their item might be a good start.
Seriously - some people just can't be helped.
Extra Detail : No, he does not mean the Eldritch Altar in Reaver's Refuge. It was a Level 7 toon talking about increasing the toughness of a weapon. Just thought I'd toss that out there before someone postulated as such.