Hello,
you need to do a hollow sphere with the inside radius parameter, here is a sample parameter:
Mandelbulb3Dv18{
g.....S....O/...w.....A....BEUvv0r32.5b4unlZRW2EvmAtC.FKJw10yw82EYJ3zojBtW/CDcyD
................................9Tbutpi4ez1........Y.RE.......................wD
...Uz6....U0..../261/.....ka....C.....E3.....YsyOS0oHNrD/26........m/dkpXm1....U
z.kAnAnD12../2.........wz.................................U0.....y1...sD...../..
.z1...sDW3siZ.xIqx1..........O77b9CN87ejR4Cxf7uxBrfM/iP7EDZRzsiKS3CN87ejlfZLVHaG
WuXZYQisYdYcykn/QD0oHNrDU.....23....2.........sD5I....sD..G.....................
.............oAnAt1...sD....zw1.........................................B....2..
.....Ksulz1.......kzAxzzz1.U..6...........EB....p..06c3..U.F....6/...I1.....SP5A
...U.qFG9yzb2zzzRYoWzz7lz1c6pc..zXCc5..UG7eFT5uj6.cIWOorVy9..0.......2.28.kFrA0.
.odIWOorVy9.2RiGMQZhz0........../EU0.wzzz1................................E.2c..
zzzz............B/...................2./8.kzzzD............8....................
.EU0.wzzz1..np0CG3Asz0kfILV56LzD.............2CcN/UvNPcveeWCNq0.yRiibHJJUk1f..XR
SvBmx3CcN/UvNPcvQsLsUa3.ibhVi1bTV1OK.sSq4uCly3CcN/UvNPcvMwLsUa3.ibhVinqTV1OK.sSq
4uCkz3CcN/UvNPcv..EsUa3feeWCNqGQIJ36wk8EwyLsUa3f.6LOZFqEcZqMfJaPms0QiR4.i.bPb/..
E6...6..F2E.....I....c.....FjFKNX34OZFaQjtKG4B3.h/........................U/4MU/
4M........U0LD8QxcE/.F8xLitRXbyD...........GdzmBjPwoz...........................
................................................................................
.....................2.....3....B....A3QcJaQZZYFH/...YYFHJ4S....................
.sU1.sE1BoU.02E...............zDnAnAnAnAnz1.....................................
...wz.........zD........kz1........wz...................kz1........wzckpX0LD8wyD
................................}
Inside a solid marble there is no verification of other objects, so they would be not shown.
The strong color change in the reflection is because the camera FOVy has 0 degrees, so the background gradient covers also 0 degrees.
In the reflection then it is only a direct line seperating top and bottom color.
But if you select 'relative to object' in the ambient tab (lighting panel), the gradient will cover always the whole background sphere, like in the parameters i shown.