Both metals are produced as by-products of lithium production: after 1958, when interest in lithium's thermonuclear properties increased sharply, the production of rubidium and caesium also increased correspondingly.
bˈoʊθ mˈɛɾəlz ɑːɹ pɹədˈuːst æz baɪpɹˈɑːdʌkts ʌv lˈɪθiəm pɹədˈʌkʃən: ˈæftɚ nˈaɪntiːnhˈʌndɹɪd fˈɪfti ˈeɪt, wˌɛn ˈɪntɹɛst ɪn lˈɪθiəmz θˈɜːmənˌuːklɪɹ pɹˈɑːpɚɾiz ɪŋkɹˈiːst ʃˈɑːɹpli, ðə pɹədˈʌkʃən ʌv ɹuːbˈɪdiəm ænd sˈiːsiəm ˈɔːlsoʊ ɪŋkɹˈiːst kˌɔːɹɪspˈɑːndɪŋli.