Doublet Gem Construction

Abstract

A doublet gem stone having a crown member of relatively high hardness and a pavilion member of lesser hardness but of higher optical dispersion and refractivity, and wherein the interface of the crown member and pavilion member is below the girdle of the gem stone. The crown member has a pavilion shoulder between the girdle and the interface which shoulder is more nearly parallel to the axis of symmetry of the gem stone than are the sides of the pavilion member and forms a projected angle of between 3.degree. and 40.degree. with said sides. The pavilion shoulder may be either partly or wholly on said crown member.

Description

BACKGROUND OF THE INVENTION

It is known that certain materials, such as strontium titanate, zircon, rutile and lithium niobate can be made in faceted crystal form to produce gem stones which rival diamond in their fire, dispersion and brilliance. Unfortunately, those materials which have appropriate optical properties to simulate diamond are generally rather soft and easily marred.

It is also known that certain other materials, such as sapphire (particularly) white sapphire), spinel, quartz, yttrium aluminum garnet (YAG) and topaz are quite hard materials which successfully resist scratching and marring under ordinary use conditions and can be made in faceted crystal form, but have poor optical properties in relation to their use as gem stones. That is, they lack the brilliance and fire of either a real diamond or one of the softer materials referred to above. Flashes of the various spectral colors are commonly called "fire" by gemologists.

It has been proposed (see, for example, U.S. Pat. No. 3,528,261 and others) to combine the good properties of both of these kinds of materials by making so-called doublet gem stones where the softer, more brilliant material is used as the lower or pavilion portion of the stone and the harder, less brilliant material is used as the upper or crown portion of the stone. Such doublets should satisfy both the physical and optical requirements of the simulated diamond market.

In point of fact, however, there is a major difficulty which has been encountered in the doublet gem field. That is, that the interface has been most difficult, if not impossible, to hide. Thus, with respect to doublet gems which have been made, in the past, for example, in accordance with the teachings of the aforementioned patent, the manufacturers have attempted to hide the interface of the crown and pavilion portions by placing this interface exactly at the girdle, or widest portion, of the doublet.

Practical experience has shown that it is substantially impossible to hide the interface of the crown and the pavilion portions even from the casual observer when such is located at the girdle.

In the effort to obviate this difficulty, it has been proposed to locate the interface below the girdle, that is between the girdle and the culet. The girdle is not intersected by the interface but instead is totally above the interface within the crown member.

Referring to FIG. 1 which illustrates this prior art approach, the crown member 1 is fabricated from a relatively hard, optically clear material such as sapphire and the pavilion member 2 is prepared from an optically brilliant and fiery material such as strontium titanate or lithium niobate. The doublet gem stone thus created has a table or flat face 8, and a girdle 3 entirely within the crown member 1 and a pavilion 2 having sides 6 tapering down to a culet 4. The crown member and pavilion member are bonded together and the bonded surface forms the interface 5 of the doublet. The sides 6 of the pavilion member are typically faceted and join with the pavilion shoulder of the crown member to form an inverted pyramid or cone.

Even though the interface 5 is moved from the girdle 3 to a location below the girdle in order to hide the interface, this relocation has created an additional problem. The interface is substantially invisible to the ordinary observer peering down at the flat face 8 of the stone. However, as can be more clearly shown in FIG. 2, the viewer can see a line 10 formed by the peripheral edge of the interface 5, this line being spaced radially inwardly at a distance D.sub.1 and from the girdle 3 of the stone. The presence of this line detracts from the aesthetic perfection of the doublet gem stone.

Furthermore, when the faceted doublet is mounted in a conventional setting such as a pronged (Tiffany) setting, the prongs contact the stone above the girdle where the stone is relatively hard and the sides of the pavilion member which is considerably softer than the crown member. This some times causes cracking or scratching of the pavilion member.

Some unitary gem stones of the prior art, exemplified by U.S. Pat. No. 3,286,486 and U.S. Pat. No. 3,490,250, show a cross-sectional configuration wherein there is a change in the angular slope of the pavilion facets between the girdle and the culet. These prior art stones, however, neither contemplated the problems presented with the doublet gem construction nor offered a solution to these problems.

BRIEF DESCRIPTION OF THE INVENTION

It is an important object of this invention to provide a novel gem stone doublet assembly.

It is another object of this invention to provide a doublet gem stone construction which more closely resembles natural real diamond than any other simulated diamond heretofore known, both from the standpoint of appearance and resistance to wear.

Other and additional objects of this invention will become apparent upon disclosure of this entire specification, including the drawings, wherein:

FIG. 1 and FIG. 2, both referred to as the prior art, refer respectively to an elevation, partly in section, of a round cut doublet gem stone and an enlarged elevation of a portion of the girdle of said gem stone; FIG. 3 is an elevation, partially in section, of a round cut doublet gem stone according to this invention;

FIG. 4 is an enlarged elevation of a portion of the gem stone shown in FIG. 3; and

FIG. 5 is an elevation partially in section of another embodiment of this invention.

In its broadest aspect, the present invention overcomes the inadequacies of the prior art doublet gem stones by positioning the interface between the hard crown member and the brilliant pavilion member at a location between the girdle and the culet, with the pavilion shoulder of the crown member more parallel to the axis of the symmetry of the stone than are the sides of the pavilion member. This axis of symmetry is an imaginary line extending normal to the plane of the table and passing through the culet of the pavilion. In this stone, the edge of the interface more closely approximates the dimensions of the girdle, thereby serving to visibly obscure the edge when the gem stone is viewed from above. In addition, this construction provides a hard surface below the girdle for contact with the metal prongs of the setting, thereby preventing the prongs from damaging the softer stone of the pavilion. The pavilion shoulder of a round cut gem stone made by this invention is more cylindrical or less conical than the sides of the pavilion. The invention is not limited, however, to round cut stones, and applies equally as well to other shaped stones.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 3 of the drawings, a round cut doublet gem stone according to this invention is composed of a crown member 11 having a relatively flat face or table 18, crown facets 19, a girdle 13, a pavilion shoulder 17 and a bottom face 15. The pavilion member 12 includes a culet 14 and conical sides 16 usually cut into suitable facets. These facets, together with the high refractivity of the pavilion member, are largely responsible for the brilliance of the gem stone. The sides 16 form an angle .DELTA. at the culet 14 of at least 90.degree.. The top of the pavilion and the bottom of the crown member are mated and bonded together to form the doublet interface 15. The color and fire of the gem stone are created mainly by the optical dispersion that occurs at the interface 15 where the two members meet. The vertical line x--x represents the axis of symmetry of the gem stone.

When, in accordance with the present invention, the pavilion shoulder 17 of the crown member 11 is made more vertical (i.e., more nearly parallel to the x--x axis) than the sides 16 of the pavilion member 12, the diameter of the peripheral edge 20 (as seen in FIG. 4) of the interface is enlarged so that it more nearly equals the maximum dimensions of the stone at the girdle. Thus, when the doublet gem stone is viewed from above, the distance D.sub.2 between the peripheral edge 20 and the girdle 13 is so small, particularly when compared with the distance D.sub.1, in the prior art stone of FIGS. 1 and 2, that the girdle tends to completely obscure the flaw created by the peripheral edge. Obviously, the distance D.sub.2 will increase as the interface 15 is moved toward the culet and away from the girdle. However, because the pavilion shoulder is more nearly parallel to the axis of symmetry than that of the prior art doublet gem stones, the distance D.sub.2 does not increase as rapidly or as noticeably as it did in the prior art stone of FIG. 1. Typically, it is desirable to keep the interface as close to the girdle as possible. However, it should be spaced a sufficient distance from the girdle to permit the prongs of the setting to contact the harder crown member both above and below the girdle.

Referring again to FIG. 3, the angle .alpha. is shown as the angle formed by the sides 16 of the pavilion member and the line representing the projection of the pavilion shoulder 17. This angle is typically between about 3.degree. and about 40.degree. and more preferably between about 10.degree. and 30.degree..

Several factors must be considered in selecting the angle .alpha., including (1) the type of stones being used for the crown member and the pavilion member, as well as their dispersions, refractive indices, hardness and brilliance; (2) the type of mounting to be used with the doublet gem stone; (3) the distance of the interface from the girdle; and (4) the method of cutting, faceting and joining the crown and pavilion. Generally, a small angle .alpha., i.e., 10-20 percent, may be used if the interface can be located close to the girdle without causing damage to the softer stone by the setting. However, if the setting contacts the doublet a substantial distance below the girdle, then the interface should be located away from the girdle and closer to the culet in which case, the angle .alpha. should be larger, i.e., between 20.degree. and 30.degree.. This larger angle serves to keep the diameter of the stone at the interface as large as possible and close to that of the girdle.

Inasmuch as the culet angle .DELTA. of the pavilion is fixed, the angle .alpha. is typically changed by altering the slope of the pavilion shoulder 17 rather than the slope of the sides 16.

As previously stated, the culet angle .DELTA. is about 90.degree.. Angle .beta. is the angle formed by the side 16 of the pavilion member and a line drawn parallel to the axis of symmetry. This angle is equal to one-half of the culet angle .DELTA./2 or about 45.degree.. The angle .alpha. approaches the angle .beta. as the pavilion shoulder 17 becomes more nearly parallel to the axis of symmetry. However, this angle .alpha. normally does not exceed 40.degree..

In an `American cut` or `ideal cut` stone, the width of the girdle measured in the direction normal to the table 18, i.e., along the gem axis x--x is generally between about 1.5 percent and about 3 percent, and more preferably about 2 percent of the diameter of the stone measured at the girdle. For this stone, the interface is preferably located at about 1 percent to 20 percent of the distance from the girdle 13 toward the culet 14. A more preferred range is between 6 percent and 15 percent of this distance. Stated another way, the total width of the girdle and the pavilion shoulder, i.e., the distance from the top of the girdle 13 to the interface 15, is typically about 6 percent of the diameter, plus or minus 0.5 percent. This represents a measurement of between about 11 percent and about 13 percent of the distance from the girdle to the culet.

FIG. 5 shows another embodiment of the invention wherein the apex of the angle .alpha. is located below the interface 15a joining the two members of the doublet gem stone. This configuration permits the use of a pavilion shoulder 17a a portion of which extends from the girdle 13a to the interface and a portion of which extends beneath the interface 20a and into the pavilion member as at 18. Also included within the scope of this invention is a doublet stone wherein the apex is located slightly above the interface. Both of these alternatives represent commercial embodiments wherein the stones are cut and faceted after being joined together, and because of inaccuracies in manufacture, the apex is not located precisely at the interface.

It is further contemplated that the girdle of the gem stone may, when desired, be faceted to enhance both the brilliance and fire of the faceted stone and to further hide the interface thereof. It is generally known to facet the girdle of real diamonds. According to the preferred aspect of this invention, faceting of the girdle improves the appearance of the gem stone, and the combination of faceting of the girdle, lowering the interface between the crown and pavilion, together with using an angular, as opposed to a straight line junction between the pavilion shoulder and the sides of the pavilion, make for a truly beautiful gem stone.

As an alternative to the faceted girdle approach, it is also within the scope of this invention to polish the girdle so that it will have the shape of a surface of revolution, e.g., a cylinder or a truncated cone. Furthermore, the girdle may be sloped in either direction, the edges may be slightly rounded and other girdle configurations may be employed without departing from the scope of the present invention.

Similarly, although the drawing hereof, shows round cut gem stones, this invention is by no means limited to this particular type of cutting. Those skilled in the art will readily appreciate its applicability to oval, square, marquise, pear or other shaped stones.

Although doubleting of gem stones is not per se claimed to be inventive, herein, it is considered desirable to point out several representative production techniques. Thus, a boule of suitable softer, brilliant fiery material, such as strontium titanate, is cut to provide one flat surface thereon. A boule of suitable harder, clear material, such as sapphire or spinel, is also cut to provide one flat surface thereon. The two flat surfaces are then juxtaposed and wringed as set forth in U.S. Pat. No. 3,528,261, or otherwise carefully cemented with a suitable, colorless, optical cement. The thus-formed doublet blank is then cut to its ultimately-desired shape and appropriately faceted in conventional manner, taking care to preserve the interface generally parallel to the top of the cut stone and to locate the interface between the girdle and the culet of the cut stone.

This method involves the obvious difficulty of cutting the pavilion shoulder at a different angle than the pavilion after the assembly of the doublet. A preferred method and one that would permit more rapid and inexpensive fabrication involves precutting and prefaceting of the crown member and the pavilion member followed by wringing or by cementing the two together.

Another approach would be to cement a prefaceted crown member to a preformed but slightly oversized pavilion member in the shape of a cone, and thereafter cutting the pavilion facets.

It has been further found, and this is a most preferred aspect of this invention, that certain doublet combinations made in the manner of the gem stone construction described herein give unusually brilliant and diamond-like appearing gem stones. These are the combination of yttrium aluminum garnet (YAG) crown and strontium titanate pavilion; yttrium aluminum garnet crown and lithium niobate pavilion; lithium niobate pavilion and spinel crown; lithium niobate pavilion and white sapphire crown; strontium titanate pavilion and white sapphire crown; and strontium titanate pavilion and spinel crown.

It has also been found that when the crown is fabricated from diamond and the pavilion is lithium niobate or strontium titanate, the nearest possible approach to a real diamond is achieved insofar as hardness, brilliance and fire are concerned. Actually, the resulting jewel has as much or more fire than a real diamond. Such a gem stone will be substantially less costly than a solid diamond of the same size because normally unusable thinner pieces of raw diamond, or slightly off-white diamond material can be used for the crown portion.

Doublet gem stones, according to this invention, should preferably utilize crown and pavilion materials which have close to the same indices of refraction. Most preferably, the indices of refraction of the doublet members should not differ from each other by more than about 0.9. Furthermore, the refractive index of the crown member should be as high as possible. Because of the ability of a highly refractive material to "bend" the rays of light, the use of a highly refractive stone for the crown member will cause the halo (the normally visible periphery of the interface) to appear very close to or immediately below the girdle. Alternatively, it permits the use of a smaller angle .alpha. or a larger distance between the girdle and the interface than that possible with a crown member made from a stone having a lower index of refraction.

It is within the spirit and scope of this invention to provide very special doublet constructions with certain types of pavilion material. Certain highly refractive materials, such as lithium niobate, for example, have two refractive indices. This material is referred to as being doubly refracting. When making gem stones of such doubly refracting material, it has been found to be most desirable to insure that the optical axis of the crystal is substantially parallel with the axis of symmetry of the cut gem stone.

When precautions are taken to insure that the lithium niobate boule is cut so that its optical axis is properly positioned, the two images of the culet, which may otherwise be observed by viewing the culet through the table, appear directly one underneath the other and very close together, whereby tending to merge the two culet images into one image.

Accurately positioning the gem axis relative to the crystal optical axis also reduces the lateral displacement of the culet facets to a minimum regardless of the angle from which the cut stone is viewed.

Other variations may be made in this invention without departing from the scope hereof, which is delimited by the claims

Claims

wherein I claim:

1. In a doublet gem stone construction comprising a crown member having a table and a girdle, and a pavilion member having a culet and joined to said crown member along an interface essentially parallel to said table and positioned between said girdle and said culet, the improvement wherein means are provided to visually obscure the edge of said interface when the stone is viewed from above, said means comprising a pavilion shoulder having at least a portion thereof on said crown member located between the girdle and the interface, the projection of said pavilion shoulder forming an angle of between 3.degree. and 40.degree. with the sides of the pavilion member therebelow, said pavilion shoulder being more nearly parallel to the axis of symmetry of the gem stone than are the sides of the pavilion member whereby the dimensions of the peripheral edge of the girdle for a given spacing between said girdle and said interface.

2. The doublet gem stone of claim 1 wherein the apex of the angle is located at the interface between the crown member and the pavilion member.

3. The gem stone of claim 1 wherein the interface is positioned at about 1 percent to 20 percent of the distance between the girdle and the culet, measured from the girdle.

4. The doublet gem stone of claim 2 wherein said angle is between about 10.degree. and about 30.degree..

5. The improved doublet gem stone of claim 4 wherein the interface is located between about 10 percent and about 15 percent of the distance between the girdle and the culet.

6. The doublet gem stone of claim 1 wherein said pavilion has a higher index of refraction and higher optical dispersion than does said crown.

7. The doublet gem stone of claim 1 wherein said crown member is of material selected from the group consisting of yttrium aluminum garnet, spinel, diamond and white sapphire and the pavilion is of material selected from the group consisting of lithium niobate and strontium titanate.

8. The doublet gem stone of claim 2 wherein said pavilion is a doubly refracting material whose optical axis is substantially parallel with the axis of symmetry of said stone.

9. The doublet gem stone of claim 1, wherein said pavilion shoulder has a truncated conical surface.