Alworthia 'Black Gem' is one of horticulture's oddities: often listed as Aloe 'Black Gem', it is a bigeneric hybrid between Aloe speciosa and Haworthia cymbiformis. It generates many tight offsets, creating a large low clump and the foliage varies from green through to dark reddish-bronzy purple with the thick triangular-conical succulent leaves becoming darker and redder the more sun it gets.

In the UK, Alworthia 'Black Gem' can be grown outside in the summer but in most regions - despite it being relatively cold-tolerant - will need to be brought indoors for the winter. A window-sill is perfect but a cold unheated greenhouse under fleece is also fine.