The emphasis tends to be on security, rather than the metal being precious.
From the 2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from
romanrite.com/girm.html :
“317. In no way should all the other things prescribed by law concerning the reservation of the Most Holy Eucharist be forgotten. [footnote 131: Cf. particularly in Sacred Congregation for the Sacraments, Instruction Nullo umquam tempore, 28 May 1938: AAS 30 (1938), pp. 198-207; Codex Iuris Canonici, cann. 934-944.”
It is unusual for a 2002 document to refer to a 1938 document, when there was a different Code of Canon Law. The 1938 document is at
RomanRite - Reservation of the Eucharist .
It includes: “4. (a) The tabernacle must be immovable and thoroughly closed. From this precept, in itself grave, the Bishop cannot dispense, nor can century-old or immemorial custom derogate, except in the case mentioned in paragraph 3. This is the first measure for the safe-keeping of the Blessed Sacrament. Absolute and complete closure necessarily demands that the tabernacle be made of solid and strong material. According to the liturgical laws, the material may be wood or marble or metal, [Footnote 3: Caeremoniale parochorum iuxta novissimas A.S. sanctiones concinnatum, art. vii. De tabernaculo, etc., n. 9 ad. 4.] but the last-mentioned material is the strongest of all. The main point is that the tabernacle be constructed of solid material, having its parts closely compacted together, and furnished with a lock strongly fixed to the door, and so designed as to ensure a completely safe locking. The hinges of the door must also be strong and well set. In some places the Bishops have prescribed that the tabernacle be entirely of metal, a measure which ensures particularly safe custody of the Blessed Sacrament and, as his Eminence Cardinal P. Gasparri teaches, [Footnote 4: De SSma Eucharistia, II, 263, n. 994.] must be absolutely observed, wherever it has been introduced. An excellent form of tabernacle is that which is a real strong iron safe, commonly known as cassaforte or coffre-fort, so that it cannot be pierced or broken by those instruments which are ordinarily used by thieves. It should be fixed by strong iron fastenings to the altar, adhering either to its lowest gradine or to the wall behind. These iron cases should be constructed either in the form of a ciborium, to be afterwards covered with marble and decorated with other ornaments, so that they exhibit the appearance of a work artistically finished, according to the terms of the second paragraph of the above-mentioned canon. Such tabernacles are called safes (Italian, di sicurezza).”
and:
“Really the use of these very solid tabernacles is a very efficacious means for the safe keeping of the Most Holy Sacrament. This Sacred Congregation does not, however, impose the burden of acquiring such tabernacles on churches which have the ordinary ones, provided these are admittedly adequate for the secure custody of the Holy Eucharist; but it recommends them for churches which are henceforth to be built. It furthermore earnestly exhorts their Lordships the Bishops, in accordance with their zeal for the Blessed Sacrament, to watch and see that the ordinary tabernacles in use throughout their dioceses have the necessary solidity to preclude every danger of sacrilegious profanation. Tabernacles which do not guarantee the absence of that danger are to be removed with the utmost severity.
5. (b) The tabernacle is to be so carefully guarded that the danger of any sort of sacrilegious profanation be excluded. It is not sufficient that a custodian reside in the place, nor is it enough that the tabernacle be so strong that it can neither be pierced by a boring instrument, nor broken open by a chisel, and is so well provided with locks that it may not be opened even with skeleton keys: a third safeguard is required by law: careful custody. Now this watchfulness, which is to be continually maintained, embraces many cautions, both ordinary and extraordinary, according to the circumstances of places and times.”
The Code of Canon Law has, in Canon 938 (at
vatican.va/archive/ENG1104/__P3C.HTM ): “§3. The tabernacle in which the Most Holy Eucharist is reserved habitually is to be immovable, made of solid and opaque material, and locked in such a way that the danger of profanation is avoided as much as possible.”
