In short; it is a magnesium-rich marble.
Dolomite is made from limestone and lime mud that come in contact with groundwater that contains magnesium.
To shop dolomite marble for jewelry making or other crafts, search under the name "Mountain jade dolomite". You can find them on Amazon, eBay, and Etsy, and a long list of independent shops from all over, so just look for the seller that suits you the best.
And here is something really cool: dolomite can be used, in medicine, to treat conditions caused by low calcium levels such as bone loss, decreased activity of the parathyroid gland, and certain muscle diseases.