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In this question, it was decided a goblin could be permanently True Polymorphed into a lump of adamantine. True Polymorph can also turn a goblin into a larger creature, like a cow. This effectively increases the mass of the goblin.

If a wizard were to turn said goblin (poor, poor Squee) into a lump of adamantine, can the wizard specify how large a lump he gets? Are there any limitations on how large it could be? For example, can the goblin become:

  1. A large boulder-sized hunk of adamantine
  2. A dragon-sized hunk of adamantine
  3. A mountain-sized hunk of adamantine
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The answer to this is in the description of the spell itself.

PHB. pg. 284 (continued from 283 True Polymorph spell description); emphasis added on the applicable point is mine

Object into Creature.

  • You can turn an object into any kind of creature, as long as the creature’s size is no larger than the object’s size and the creature’s challenge rating is 9 or lower.

This stands to reason that this limitation is applied both ways. Although the Creature to Object section doesn't explicitly cover it, since Object into Creature does cover the size limitation literally right before it, it stands to reason that changing a creature into an object limits the size of the object to the equivalent mass of a creature within the CR 9 rating limitation. So if you wanted to turn a chipmunk into a house, it wouldn't fly. In addition, this prevents the abuse of turning a mountain into a mouse, and then letting the spell wear off inside a town you don't like anymore.

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    \$\begingroup\$ "In addition, this prevents the abuse of turning a mountain into a mouse, and then letting the spell wear off inside a town you don't like anymore." <-- actually I don't think it does -- Object into Creature allows one to polymorph an object into a smaller creature (mountain to mouse). \$\endgroup\$ – PurpleVermont Jul 10 '15 at 4:56
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    \$\begingroup\$ There's nothing with a CR of 9 or less that's the size of a mountain. If there was no upper limit on this, than what's to stop a player from turning the planet into a gold coin and destroying the world? \$\endgroup\$ – Lino Frank Ciaralli Jul 10 '15 at 11:20
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    \$\begingroup\$ Frank: the answer to the rhetorical, "what's to stop" is "The DM's authority." :) I liked your answer enough to up vote it. \$\endgroup\$ – KorvinStarmast Jul 10 '15 at 12:52
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    \$\begingroup\$ As a DM, I would also shut down any attempt to transform a mountain into anything, on the basis of a mountain not being an object, but rather a collection of many thousands of objects, including many different forms of vegetation, thousands of different rocks, water from streams, etc. \$\endgroup\$ – Taejang Jul 13 '15 at 14:15
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    \$\begingroup\$ @Dan True. But there are single-object geological features much larger than mere mountains. Have fun with that mouse! :) \$\endgroup\$ – Please stop being evil Jul 14 '15 at 6:37
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The simple answer is that this is in the realm of DM authority. How much change in mass the DM is willing to put up with is a DM decision.

Example 1:
If someone wants to true polymorph Saint Helen into a mountain, and DM would like a Mount Saint Helen to be based in that origin for the name, then maybe the DM gives the OK.

Example 2:
If a player wants to true polymorph Saint Helen into a mountain sized pile of pure platinum, DM just might roll eyes and not allow it.

A way to answer this question from a game mechanics perspective is:

What is the CR of a mountain?

This borrows from the "Creature to Creature" description on p. 283 of PHB, but I think this gives at least a guideline for bounding the problem.

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