Crassula rupestris subsp. commutata (Friedr.) Toelken
Crassula rupestris subsp. commutata is a much-branched, succulent subshrub that grows up to 12 inches (30 cm) tall. The stems are thin, fleshy near the growing tip, but rather woody lower down. The leaves are grayish (during the summer months the leaf margins turn a brilliant red or yellow), hardly fused and never broader than 0.16 inch (4 mm). It is easily confused with Crassula brevifolia but can be distinguished by its spreading leaves not longer than 0.4 inches (1 cm). Flowers are grouped in clusters, white to pale pink, star-shaped, and up to 0.25 inch (6 mm) across.
USDA hardiness zone 9b to 11b: from 25 °F (−3.9 °C) to 50 °F (+10 °C).
Crassulas are easy to grow, but they are susceptible to mealy bugs and fungal diseases. As with all succulents, overwatering is sure to be fatal, so err on the side of too dry rather than too wet. Never let your Crassula sit in water. If you water from beneath by letting the plant sit in a saucer of water, make sure to pour off any excess water after a few minutes.
Crassulas are generally started by division, offsets, or leaf cuttings. Plants can be easily propagated from a single leaf: sprout leaves by placing them into a succulent or cacti mix, then covering the dish until they sprout.
Repot as needed, preferably during the warm season. To repot a succulent, make sure the soil is dry before repotting, then gently remove the pot. See more at How to Grow and Care for Crassula.
Native to Namibia and South Africa.
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'Tom Thumb' (Crassula rupestris ssp. commutata) (Tölken): A miniature stacked Crassula. This dwarf succulent originated in Namibia and South Africa and is well-adapted to hot, dry rocky slopes. Its thick, triangular leaves stay under 0.4" long but store enough water to help the plant endure extended drought.
'Tom Thumb' grows best in bright sunlight and the light helps its leaves show their fine red margins. It thrives in well-draining pots and gritty soil, only occasionally needing a deep watering when the soil is fully dry.
This freely branching shrublet will stay small in a pot, though it can eventually reach over 6.0" long and spill beautifully from arrangements. From spring to mid-summer, 'Tom Thumb' can display neat clusters of tiny white flowers.
Crassula can show tiny white or black dots on their leaves. These are healthy, water-transferring pores called "hydathodes" and not a sign of disease.
Crassula rupestris, called buttons on a string, is a species of Crassula native to the Cape Provinces of South Africa.  It has gained the Royal Horticultural Society's Award of Garden Merit.  It is also called bead vine, necklace vine, and rosary vine. 
The following subspecies are currently accepted: 
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Accepted Scientific Name: Crassula rupestris subs. commutata (Friedrich) Toelken
J. S. African Bot. 41: 116 1975
Subspecies, varieties, forms and cultivars of plants belonging to the Crassula rupestris group
Bibliography: Major references and further lectures
1) John Wilkes “Encyclopaedia Londinensis” Volume 5 1810
2) George Don, Philip Miller “A General System of Gardening and Botany: Founded Upon Miller's Gardener's Dictionary, and Arranged According to the Natural System” Volume 3 C. J. G. and F. Rivington, 1834
3) Werner Rauh “The Wonderful World of Succulents: Cultivation and Description of Selected Succulent Plants Other Than Cacti” Smithsonian Institution Press, 1984
4) Doreen Court “Succulent Flora of Southern Africa” CRC Press, 01/giu/2000
5) Stuart Max Walters “The European Garden Flora: Dicotyledons” (Part I) Cambridge University Press, 1989
6) Gordon D. Rowley “The illustrated encyclopedia of succulents” Crown Publishers, 01/Aug/1978
7) Urs Eggl “Sukkulenten-Lexikon. Crassulaceae (Dickblattgewächse).” Eugen Ulmer, Stuttgart 2003
8) Hermann Jacobsen “Abromeitiella to Euphorbia” Blandford Press, 1960
9) Hermann Jacobsen “A handbook of succulent plants: descriptions, synonyms, and cultural details for succulents other than Cactaceae” Volume 1 Blandford Press, 1960
10) Toelken, H.R. 1997. “A revision of the genus Crassula” in southern Africa. Annals of the Bolus Herbarium 8,1-595.
11) Dr J.P. Roux “Flora of South Africa” 2003
12) Van Jaarsveld, E., Van Wyk, B-E. & Smith, G. “Succulents of South Africa.” Tafelberg, Cape Town. 2000
13) John Manning “Field Guide to Fynbos” Struik, 2007
14) Reader's Digest Association “A-Z of rock garden & water plants” Reader's Digest Association, 01/Dec/1995
15) Vera Higgins “Succulent Plants Illustrated” Blandford Press, 1949
16) Leon Nell “The Garden Route and Little Karoo” Struik Publishers, 01/Jan/2003
17) Domitilla Raimondo “Red list of South African plants 2009” South African National Biodiversity Institute, 2009