Metallurgical speaking Tamahagane is a steel ore made from iron sand and charcoal (carbon). These two materials are smelted in a bloomery also known as a tatara. The iron and carbon elements will smelt together into a steel ore known as tamahagane.
There are really just of few tatara left of which the most important is located in Yokota Village, Shimane prefecture. The process is under strict supervision of the NBTHK (Nittoho) organisation, who also control the sales and allocation of the ore.
The 'Nittoho Tatara' is lit only in the deep winter, when the air is at its driest. During this period of around three weeks it's possible to create maximum of three batches of Tamahagane, since the process takes 7 days to complete.
Because of this limited supply of Tamahagane, prices are high and many starting sword smiths cannot afford it. For this reason many of them also use a blend called orishigane (left-over shavings steel). They use old nails, shavings and any other iron containing material as a basis for the steel. By adding (or excluding) charcoal and straw to their - often self made - mini tatara they can control the carbon content of their Oroshigane steel ore.
Since the process of Tamahagane production is so heavy regulated by the government and the NBTHK organisation it is not possible for non-registered sword smith to use or purchase Nittoho Tamahagane.
Luckily, outside of Japan there also aren't any rules for building a tatara and making your own tamahagane. Important is to know that the iron sand used in Nittoho tamahagane (satetsu ~ dark green/iron colored sand) can only be found in the very north of Japan. The impurity and exact chemical contents of this sand determine the visual details of Tamahagane.
This means that Tamahagane steel made of other iron sand may show some different coloration and features than 'true Tamahagane' made of satetsu sand.