This element has rarely been suggested and the few people who have suggeted it never gave it any real uses.  But I'm gonna be different, so here it is:

Coral (or CRAL):

CRAL is a solid that has almost all the opposite properties as PLNT.  It turns OXYG into CO2, it grows with SLTW but turns into DCRL (see below) with WATR ACID or CAUS (CAUS is just gaseous ACID,) and whereas neutrons turn PLNT into WOOD, they turn CRAL into METL (in real life metal is depicted as the opposite of wood.)  Here's another element below that provides even further realism (tpt is 75-80% realistic, gotta keep it that way):

Dead coral (or DCRL):

DCRL is a powder.  To make it, CRAL (see above) must absorb WATR, ACID, or CAUS.  DCRL is immune to ACID and CAUS.  Under a pressure of 5 or higher it has 2/3 of a chance of turning into OIL and 1/3 of a chance of turning into GAS.  Brush-spawned DCRL turns into CRAL upon contact with CRAL.

NOTE:  While CRAL is alive (as is PLNT, YEST, TRON, STKM, STK2, FIGH, and LIFE,) it is not meant to be part of a biology simulator as that's not what tpt is.  That being said, this element is an animal; albiet a stationary one, and can be used to simulate respiration without CONV while the other element simulates the formation of fossil fuels.  People can construct gills or lungs out of CRAL and realistic oil/natural gas fields using DCRL.  Currently only VACU, VENT, BHOL, and WHOL have "opposite" elements.  We should give another element (like PLNT) one.