Goddess Amber is a type of stone used by the druids to store various mystical elements. One type of stone is specifically made to seal demonic beings within a radius of a few feet of it. The goddess amber was created by Merlin.
They resemble diamonds and other precious crystals.
The goddess amber is first presented by Hendrickson, who requests Veronica Liones to utilize is to seal Meliodas. Stating that is it a present from their father, Veronica easily delivers it to Elizabeth, who mistakenly leaves it with Meliodas, allowing Veronica to chant the incantation and seal him. One giant amber was used 10 years ago by Merlin and the druids on Meliodas to seal the bulk of his demonic power. They are also used to store various monsters to be used for training purposes in the druid homeland.
They seem to mainly function to store living creatures and objects. One specific amber owned by Hendrickson and given to Veronica was capable of sealing those with demonic powers within a radius of a few feet. Another, far larger one, was used to contain Meliodas stolen demonic power. According to Merlin, they can only be used to seal Red Demons and Gray Demons, which are the weakest demons within the clan.