The large buckle and buttons are celluloid, the smallest glass and the middle MOP.
If you love dog buttons, see also the posts from 26th April, 6th May 2018, 28th June 2018 and 14th April 2019.
Why are Scotty dogs so popular? An article from a pet website has some information: https://www.terrificpets.com/articles/10210965.asp
An British artist who moved to America, Marguerite Kirmse, was very successful drawing animals, dogs in particular, including Scotties, in the 1920s.
One early example in pop-culture were “Ric and Rac”, a Wire Fox terrier and his Scottish terrier buddy in the 1930s. They were carton characters created by Paul Abraham (professional name Pol Rab) that featured in a French newspaper. They were so popular that their images were used for brooches, housewares and ornaments. As scotch terrier/fox terrier style buttons date from the late 1930s, they may have been an inspiration.
The famous pet of Franklin Roosevelt, Fala, further cemented these iconic dogs into our consciousness.