The least likely of the red seaweeds are the crusty Lithothamnions (Corallina order but of a different family) that look like splatters of pink paint blistered onto random stones in rock pools. Crustose, like their landlocked friends the lichens, they are surprisingly difficult to prise off their homes, although you may be lucky if you find one who has attached itself to another algae.
Lithothamnions do not resemble our concept of seaweeds, although like their leafy cousins, they are in fact composed of leaves; here as single leaves hinged together to form a hard limestone layer. This makes them impervious to various tide pool grazers such as sea urchins (who prefer big kelp holdfasts in any case) and allows them to colonise when areas have been cleared, creating an essential building block of the coral reefs.
As calcareous organisms, Lithothamnions are a sought after source of calcium. They, in tandem with other red algae Corallines (such as Phymatolithon calcareum), have been dredged and harvested for agricultural use for hundreds of years; a practice which is now banned in the UK and Ireland as the beds take so long to regrow. However, smaller quantities are being harvested sustainably as marine calcium can be assimilated by our skin 100 x more efficiently than mineral calcium and brings with it a host of trace minerals including phosphorus, potassium, manganese, boron, iodine, copper, selenium, cobalt and zinc.
For this reason, different species are used in bone-replacement therapies as well as serving as an active ingredient in modern-day spa treatments, often in combination with other seaweeds. Dried, ground and powdered they can also be ingested (usually in combination with kelps) to help disperse fatty cells and break down cellulite, whilst absorbing toxins and helping to discharge them from the system.