Does the jade circlet worth 1,500 gp (the material component of the spell) get consumed every time you cast shapechange? Or does it remain and you must simply be wearing it in order to cast the spell?
Spells that consume their material components clearly say so in the component section of their description. Eg. the spell "Reincarnate" says the following:
rare oils and unguents worth at least 1,000 gp, which the spell consumes
Since the circlet is not said to be consumed in that manner, it's left unconsumed.
The Jade circlet is not consumed
Spell components are only consumed if they say that they are.
See for instance, Revivify:
Components: V, S, M (diamonds worth 300 gp, which the spell consumes)
As opposed to Shapechange:
Components: V, S, M (a jade circlet worth at least 1,500 gp, which you must place on your head before you cast the spell)
No, it is not consumed
Material components are only consumed if the spell description specifically says they are.
The Shapechange spell only says that you must have a jade circlet that you wear on your head when casting the spell.
Contrast this with, for example, Raise Dead which says that you need:
a diamond worth at least 500gp, which the spell consumes
The spell does not say it is consumed, so it isn't. You just have to place it on your head before you cast the spell.